Rf filter for an active medical device (amd) for handling high rf power induced in an associated implanted lead from an external rf field

ABSTRACT

An AIMD includes a conductive housing, an electrically conductive ferrule with an insulator hermetically sealing the ferrule opening. A conductive pathway is hermetically sealed and disposed through the insulator. A filter capacitor is disposed within the housing and has a dielectric body supporting at least two active and two ground electrode plates interleaved, wherein the at least two active electrode plates are electrically connected to the conductive pathway on the device side, and the at least two ground electrode plates are electrically coupled to either the ferrule and/or the conductive housing. The dielectric body has a dielectric constant less than 1000 and a capacitance of between 10 and 20,000 picofarads. The filter capacitor is configured for EMI filtering of MRI high RF pulsed power by a low ESR, wherein the ESR of the filter capacitor at an MRI RF pulsed frequency or range of frequencies is less than 2.0 ohms.

CROSS-REFERENCE TO RELATED APPLICATIONS

The instant application is a continuation-in-part application claimingpriority to application Ser. No. 14/187,297 filed on Feb. 23, 2014,which itself was a continuation application claiming priority toapplication Ser. No. 14/088,849 filed on Nov. 25, 2013, which itself wasa continuation application of Ser. No. 13/408,020 filed on Feb. 29,2012, which itself claimed priority to provisional application61/448,069 filed on Mar. 1, 2011, the contents of all of which are fullyincorporated herein with these references.

DESCRIPTION Field of the Invention

This invention generally relates to the problem of RF energy inducedinto implanted leads during medical diagnostic procedures such asmagnetic resonant imaging (MRI), and provides methods and apparatus forredirecting RF energy to locations other than the distal tipelectrode-to-tissue interface. In addition, the present inventionprovides electromagnetic interference (EMI) protection to sensitiveactive implantable medical device (AIMD) electronics.

BACKGROUND OF THE INVENTION

Compatibility of cardiac pacemakers, implantable defibrillators andother types of active implantable medical devices with magneticresonance imaging (MRI) and other types of hospital diagnostic equipmenthas become a major issue. If one proceeds to the websites of the majorcardiac pacemaker manufacturers in the United States, which include St.Jude Medical, Medtronic and Boston Scientific (formerly Guidant), onewill see that the use of MRI is generally contra-indicated for patientswith implanted pacemakers and cardioverter defibrillators. See alsorecent press announcements of the Medtronic Revo MRI pacemaker which wasrecently approved by the U.S. FDA. With certain technical limitations asto scan type and location, this is the first pacemaker designed for MRIscanning. See also:

-   (1) “Safety Aspects of Cardiac Pacemakers in Magnetic Resonance    Imaging”, a dissertation submitted to the Swiss Federal Institute of    Technology Zurich presented by Roger Christoph Luchinger, Zurich    2002;-   (2) “1. Dielectric Properties of Biological Tissues: Literature    Survey”, by C. Gabriel, S. Gabriel and E. Cortout;-   (3) “II. Dielectric Properties of Biological Tissues: Measurements    and the Frequency Range 0 Hz to 20 GHz”, by S. Gabriel, R. W. Lau    and C. Gabriel;-   (4) “III. Dielectric Properties of Biological Tissues: Parametric    Models for the Dielectric Spectrum of Tissues”, by S. Gabriel, R. W.    Lau and C. Gabriel;-   (5) “Advanced Engineering Electromagnetics”, C. A. Balanis, Wiley,    1989;-   (6) Systems and Methods for Magnetic-Resonance-Guided Interventional    Procedures, Patent Application Publication US 2003/0050557, Susil    and Halperin et al., published Mar. 13, 2003;-   (7) Multifunctional Interventional Devices for MRI: A Combined    Electrophysiology/MRI Catheter, by, Robert C. Susil, Henry R.    Halperin, Christopher J. Yeung, Albert C. Lardo and Ergin Atalar,    MRI in Medicine, 2002; and-   (8) Multifunctional Interventional Devices for Use in MRI, U.S. Pat.    No. 7,844,534, Susil et al., issued Nov. 30, 2010.    The contents of the foregoing are all incorporated herein by    reference.

However, an extensive review of the literature indicates that, despitebeing contra-indicated, MRI is indeed often used to image patients withpacemaker, neurostimulator and other active implantable medical devices(AIMDs). As such, the safety and feasibility of MRI in patients withcardiac pacemakers is an issue of gaining significance. The effects ofMRI on patients' pacemaker systems have only been analyzedretrospectively in some case reports. There are a number of papers thatindicate that MRI on new generation pacemakers can be conducted up to0.5 Tesla (T). MRI is one of medicine's most valuable diagnostic tools.MRI is, of course, extensively used for imaging, but is also used forinterventional medicine (surgery). In addition, MRI is used in real timeto guide ablation catheters, neurostimulator tips, deep brain probes andthe like. An absolute contra-indication for pacemaker or neurostimulatorpatients means that these patients are excluded from MRI. This isparticularly true of scans of the thorax and abdominal areas. Because ofMRI's incredible value as a diagnostic tool for imaging organs and otherbody tissues, many physicians simply take the risk and go ahead andperform MRI on a pacemaker patient. The literature indicates a number ofprecautions that physicians should take in this case, including limitingthe power of the MRI RF pulsed field (Specific Absorption Rate—SARlevel), programming the pacemaker to fixed or asynchronous pacing mode,and then careful reprogramming and evaluation of the pacemaker andpatient after the procedure is complete. There have been reports oflatent problems with cardiac pacemakers or other AIMDs after an MRIprocedure sometimes occurring many days later. Moreover, there are anumber of recent papers that indicate that the SAR level is not entirelypredictive of the heating that would be found in implanted leads ordevices. For example, for magnetic resonance imaging devices operatingat the same magnetic field strength and also at the same SAR level,considerable variations have been found relative to heating of implantedleads. It is speculated that SAR level alone is not a good predictor ofwhether or not an implanted device or its associated lead system willoverheat.

There are three types of electromagnetic fields used in an MRI unit. Thefirst type is the main static magnetic field designated B₀ which is usedto align protons in body tissue. The field strength varies from 0.5 to3.0 Tesla in most of the currently available MRI units in clinical use.Some of the newer MRI system fields can go as high as 4 to 5 Tesla. Atthe International Society for Magnetic Resonance in Medicine (ISMRM),which was held on 5-6 Nov. 2005, it was reported that certain researchsystems are going up as high as 11.7 Tesla and will be ready sometime in2010. This is over 100,000 times the magnetic field strength of theearth. A static magnetic field can induce powerful mechanical forces andtorque on any magnetic materials implanted within the patient. Thiswould include certain components within the cardiac pacemaker itselfand/or lead systems. It is not likely (other than sudden system shutdown) that the static MRI magnetic field can induce currents into thepacemaker lead system and hence into the pacemaker itself. It is a basicprinciple of physics that a magnetic field must either be time-varyingas it cuts across the conductor, or the conductor itself must movewithin a specifically varying magnetic field for currents to be induced.

The second type of field produced by magnetic resonance imaging is thepulsed RF field which is generated by the body coil or head coil. Thisis used to change the energy state of the protons and elicit MRI signalsfrom tissue. The RF field is homogeneous in the central region and hastwo main components: (1) the electric field is circularly polarized inthe actual plane; and (2) the H field, sometimes generally referred toas the net magnetic field in matter, is related to the electric field byMaxwell's equations and is relatively uniform. In general, the RF fieldis switched on and off during measurements and usually has a frequencyof about 21 MHz to about 500 MHz depending upon the static magneticfield strength. The frequency of the RF pulse for hydrogen scans variesby the Lamour equation with the field strength of the main static fieldwhere: RF PULSED FREQUENCY in MHz=(42.56) (STATIC FIELD STRENGTH INTESLA). There are also phosphorous and other types of scanners whereinthe Lamour equation would be different. The present invention applies toall such scanners.

The third type of electromagnetic field is the time-varying magneticgradient fields designated B_(X), B_(Y), B_(Z), which are used forspatial localization. These change their strength along differentorientations and operating frequencies on the order of 2-5 kHz. Thevectors of the magnetic field gradients in the X, Y and Z directions areproduced by three sets of orthogonally positioned coils and are switchedon only during the measurements. In some cases, the gradient field hasbeen shown to elevate natural heart rhythms (heart beat). This is notcompletely understood, but it is a repeatable phenomenon. The gradientfield is not considered by many researchers to create any other adverseeffects.

It is instructive to note how voltages and electro-magnetic interference(EMI) are induced into an implanted lead system. At very low frequency(VLF), voltages are induced at the input to the cardiac pacemaker ascurrents circulate throughout the patient's body and create voltagedrops. Because of the vector displacement between the pacemaker housingand, for example, the tip electrode, voltage drop across the resistanceof body tissues may be sensed due to Ohms Law and the circulatingcurrent of the RF signal. At higher frequencies, the implanted leadsystems actually act as antennas where voltages (EMFs) are induced alongtheir length. These antennas are not very efficient due to the dampingeffects of body tissue; however, this can often be offset by extremelyhigh power fields (such as MRI pulsed fields) and/or body resonances. Atvery high frequencies (such as cellular telephone frequencies), EMIsignals are induced only into the first area of the leadwire system (forexample, at the header block of a cardiac pacemaker). This has to dowith the wavelength of the signals involved and where they coupleefficiently into the system.

Magnetic field coupling into an implanted lead system is based on loopareas. For example, in a cardiac pacemaker unipolar lead, there is aloop formed by the lead as it comes from the cardiac pacemaker housingto its distal tip electrode, for example, located in the rightventricle. The return path is through body fluid and tissue generallystraight from the tip electrode in the right ventricle back up to thepacemaker case or housing. This forms an enclosed area which can bemeasured from patient X-rays in square centimeters. The average looparea is 200 to 225 square centimeters. This is an average and is subjectto great statistical variation. For example, in a large adult patientwith an abdominal pacemaker implant, the implanted loop area is muchlarger (around 400 square centimeters).

Relating now to the specific case of MRI, the magnetic gradient fieldswould be induced through enclosed loop areas. However, the pulsed RFfields, which are generated by the body coil, would be primarily inducedinto the lead system by antenna action. Subjected to RF frequencies, thelead itself can exhibit complex transmission line behavior.

At the frequencies of interest in MRI, RF energy can be absorbed andconverted to heat. The power deposited by RF pulses during MRI iscomplex and is dependent upon the power (Specific Absorption Rate (SAR)Level) and duration of the RF pulse, the transmitted frequency, thenumber of RF pulses applied per unit time, and the type of configurationof the RF transmitter coil used. The amount of heating also depends uponthe volume of tissue imaged, the electrical resistivity of tissue andthe configuration of the anatomical region imaged. There are also anumber of other variables that depend on the placement in the human bodyof the AIMD and the length and trajectory of its associated lead(s). Forexample, it will make a difference how much EMF is induced into apacemaker lead system as to whether it is a left or right pectoralimplant. In addition, the routing of the lead and the lead length arealso very critical as to the amount of induced current and heating thatwould occur. Also, distal tip design is very important as it can heat updue to MRI RF induced energy.

The cause of heating in an MRI environment is twofold: (a) RF fieldcoupling to the lead can occur which induces significant local heating;and (b) currents induced between the distal tip and tissue during MRI RFpulse transmission sequences can cause local Ohms Law (resistive)heating in tissue next to the distal tip electrode of the implantedlead. The RF field of an MRI scanner can produce enough energy to induceRF voltages in an implanted lead and resulting currents sufficient todamage some of the adjacent myocardial tissue. Tissue ablation(destruction resulting in scars) has also been observed. The effects ofthis heating are not readily detectable by monitoring during the MRI.Indications that heating has occurred would include an increase inpacing threshold, venous ablation, Larynx or esophageal ablation,myocardial perforation and lead penetration, or even arrhythmias causedby scar tissue. Such long term heating effects of MRI have not been wellstudied yet for all types of AIMD lead geometries. There can also belocalized heating problems associated with various types of electrodesin addition to tip electrodes. This includes ring electrodes or padelectrodes. Ring electrodes are commonly used with a wide variety ofimplanted devices including cardiac pacemakers, and neurostimulators,and the like. Pad electrodes are very common in neurostimulatorapplications. For example, spinal cord stimulators or deep brainstimulators can include at least ten pad electrodes to make contact withnerve tissue. A good example of this also occurs in a cochlear implant.In a typical cochlear implant there would be sixteen pad electrodesplaced up into the cochlea. Several of these pad electrodes make contactwith auditory nerves.

Although there are a number of studies that have shown that MRI patientswith active implantable medical devices, such as cardiac pacemakers, canbe at risk for potential hazardous effects, there are a number ofreports in the literature that MRI can be safe for imaging of pacemakerpatients when a number of precautions are taken (only when an MRI isthought to be an absolute diagnostic necessity). While these anecdotalreports are of interest, they are certainly not scientificallyconvincing that all MRI can be safe. For example, just variations in thepacemaker lead length and implant trajectory can significantly affecthow much heat is generated. A paper entitled, HEATING AROUNDINTRAVASCULAR GUIDEWIRES BY RESONATING RF WAVES by Konings, et al.,journal of Magnetic Resonance Imaging, Issue 12:79-85 (2000), does anexcellent job of explaining how the RF fields from MRI scanners cancouple into implanted leads. The paper includes both a theoreticalapproach and actual temperature measurements. In a worst-case, theymeasured temperature rises of up to 74 degrees C. after 30 seconds ofscanning exposure. The contents of this paper are incorporated herein byreference.

The effect of an MRI system on the function of pacemakers, ICDs,neurostimulators and the like, depends on various factors, including thestrength of the static magnetic field, the pulse sequence, the strengthof RF field, the anatomic region being imaged, and many other factors.Further complicating this is the fact that each patient's condition andphysiology is different and each lead implant has a different lengthand/or implant trajectory in body tissues. Most experts still concludethat MRI for the pacemaker patient should not be considered safe.

It is well known that many of the undesirable effects in an implantedlead system from MRI and other medical diagnostic procedures are relatedto undesirable induced EMFs in the lead system and/or RF currents in itsdistal tip (or ring) electrodes. This can lead to overheating of bodytissue at or adjacent to the distal tip.

Distal tip electrodes can be unipolar, bipolar and the like. It is veryimportant that excessive current not flow at the interface between thelead distal tip electrode and body tissue. In a typical cardiacpacemaker, for example, the distal tip electrode can be passive or of ascrew-in helix type as will be more fully described. In any event, it isvery important that excessive RF current not flow at this junctionbetween the distal tip electrode and for example, myocardial or nervetissue. Excessive current at the distal electrode to tissue interfacecan cause excessive heating to the point where tissue ablation or evenperforation can occur. This can be life threatening for cardiacpatients. For neurostimulator patients, such as deep brain stimulatorpatients, thermal injury can cause coma, permanent disability or even belife threatening. Similar issues exist for spinal cord stimulatorpatients, cochlear implant patients and the like.

Interestingly, the inventors performed an experiment in an MRI scannerwith a human body gel-filled phantom. In the phantom, placed in ananatomic position, was an operating pacemaker and a lead. This wasduring evaluation of the efficacy of bandstop filters at or near thedistal tip electrode for preventing the distal tip electrode fromoverheating. Bandstop filters for this purpose are more thoroughlydescribed in U.S. Pat. No. 7,363,090, the contents of which areincorporated herein by reference. During the experiments, there was acontrol lead that had no bandstop filter. During a particularly RFintense scanning sequence, Luxtron probes measured a distal helix tipelectrode temperature rise of 30 degrees C. Of course, the 30 degrees C.temperature rise in a patient, would be very alarming as it could leadto pacing capture threshold changes or even complete loss capture due toscar tissue formation. An identical lead with the bandstop filter inplace only had a temperature rise of 3 degrees C. This was a remarkablevalidation of the efficacy of bandstop filters for implantableelectrodes. However, something very interesting happened when wedisconnected the pacemaker. We disconnected the pacemaker and put asilicone lead cap over the proximal end of the lead. Again, we put thegel phantom back inside the MR scanner and this time we measured an 11degree C. temperature rise on the lead with the bandstop filter. Thiswas proof positive that the housing of the AIMD acts as part of thesystem. The prior art feedthrough capacitor created a fairly lowimpedance at the input to the pacemaker and thereby drew RF energy outof the lead and diverted it to the housing of the pacemaker. It hasrecently been discovered that the impedance, and in particular, the ESRof these capacitors, is very important so that maximal energy can bepulled from the lead and diverted to the pacemaker housing while at thesame time, not unduly overheating the feedthrough capacitor.

Accordingly, there is a need for novel low ESR diverting capacitors andcircuits which are frequency selective and are constructed of passivecomponents for implantable leads and/or leadwires. Further, there is aneed for very low ESR diverter element capacitor(s) which are designedto decouple a maximum amount of induced RF energy from an implanted leadto an AIMD housing while at the same time not overheat. The presentinvention fulfills these needs and provides other related advantages.

SUMMARY OF THE INVENTION

An embodiment of an active implantable medical device (AIMD) includes aconductive housing. The housing defines a body fluid side locatedoutside the conductive housing and defines a device side located insidethe conductive housing. An electrically conductive ferrule ishermetically sealed to a housing opening in the conductive housing. Theferrule has a ferrule opening passing through the ferrule between thebody fluid side and the device side. An insulator hermetically seals theferrule opening. A conductive pathway is hermetically sealed anddisposed through the insulator between the body fluid side and thedevice side. The conductive pathway is in non-conductive relation withthe ferrule. The conductive pathway on the body fluid side isconnectable to an implantable lead conductor with at least oneelectrode. A filter capacitor is disposed within the conductive housingon the device side. The filter capacitor includes a dielectric bodysupporting at least two active electrode plates interleaved with atleast two ground electrode plates, wherein the at least two activeelectrode plates are electrically connected to the conductive pathway onthe device side, and the at least two ground electrode plates areelectrically coupled to either the ferrule and/or the conductivehousing. The filter capacitor also has a dielectric body with adielectric constant less than 1000. A capacitance is between 10 and20,000 picofarads. The filter capacitor is configured for EMI filteringof MRI high RF pulsed power by a low equivalent series resistance (ESR),wherein the ESR is the sum of a dielectric loss plus an ohmic loss,wherein the ESR of the filter capacitor at an MRI RF pulsed frequency orrange of frequencies is less than 2.0 ohms.

The filter capacitor may be selected from the group consisting of amonolithic ceramic capacitor, a flat-through capacitor, a chip capacitorand an X2Y attenuator.

A circuit board or substrate may be disposed on the device side, whereinthe filter capacitor is mounted to the circuit board or substrate andthe circuit board or substrate is located inside the conductive housingon the device side of the AIMD. The circuit board or substrate may bedisposed adjacent and attached to either the ferrule and/or theinsulator. Or, the circuit board or substrate may be disposed distantfrom and unattached to either the ferrule and/or the insulator.

The filter capacitor may be the first filter capacitor connected to theconductive pathway disposed on the device side. Said differently, thereare no other filters capacitors or electronic circuits containing filtercapacitors in the conductive pathway between and to the insulatorhermetically sealing the ferrule opening and the filter capacitor. Aninductor or inductance may be disposed between the filter capacitor andthe at least one electrode of the implantable lead conductor.

A diode or back-to-back diodes may be connected at a first diode end tothe conductive pathway and connected at a second diode end to either theferrule and/or the conductive housing. Or a diode or back-to-back diodesmay be connected in parallel with the filter capacitor to the conductivepathway and to either the ferrule and/or the conductive housing.

The filter capacitor may be a first capacitor element of a multielementbroadband lowpass filter having at least one inductor, the multielementbroadband lowpass filter forming one of the group consisting of areverse L filter, an LL, a Pi and an n-element lowpass filter.

The at least two active electrode plates of the filter capacitor may beelectrically connected to the conductive pathway devoid of anyintermediate electronic circuits or filters disposed in series betweenthe conductive pathway and the at least two active electrode plates.

The filter capacitor may be an element of a multielement broadbandlowpass filter having at least one inductor, the multielement broadbandlowpass filter forming one of the group consisting of an L filter, areverse L filter, an LL, a reverse LL, a T, a Pi and an n-elementlowpass filter.

A dielectric loss tangent measured in ohms at the MRI RF pulsedfrequency or range of frequencies may be less than five percent of thefilter capacitor's ESR. The filter capacitor may be a passive componentlowpass filter.

The MRI RF pulsed frequency or range of frequencies may include 64 MHzor 128 MHz. The capacitance may be between 350 and 10,000 picofarads.The filter capacitor's ESR at the MRI RF pulsed frequency or range offrequencies may be less than 0.5 ohms or 0.1 ohms. The dielectricconstant may be less than 1000, 200 or 90.

In other embodiments the at least two active and at least two groundelectrode plates may also be at least five, ten, twenty, forty or moreactive and ground plates.

An insulative washer may be placed or located between the filtercapacitor and then also either the insulator and/or ferrule.

Another embodiment of an active implantable medical device (AIMD)includes a conductive housing. The conductive housing defines a bodyfluid side outside the conductive housing and defines a device sideinside the conductive housing. An electrically conductive ferrule ishermetically sealed to a housing opening in the conductive housing. Theferrule has a ferrule opening passing through the ferrule between thebody fluid side and the device side. An insulator hermetically seals theferrule opening. A conductive pathway hermetically seals and is disposedthrough the insulator between the body fluid side and the device side.The conductive pathway is in non-conductive relation with the ferrule.The conductive pathway on the body fluid side is connectable to animplantable lead conductor, wherein the implantable lead conductor hasat least one electrode configured to be contactable to biological cells.A filter capacitor is disposed within the conductive housing on thedevice side. The filter capacitor includes a dielectric body supportingat least two active electrode plates interleaved with at least twoground electrode plates, wherein the at least two active electrodeplates are electrically connected to the conductive pathway, and the atleast two ground electrode plates are electrically coupled to either theferrule and/or the conductive housing. The dielectric body has adielectric constant less than 1000. A capacitance is between 10 and20,000 picofarads. The filter capacitor is configured for EMI filteringof MRI high RF pulsed power by a low equivalent series resistance (ESR),wherein the ESR is the sum of a dielectric loss plus an ohmic loss,wherein the ESR of the filter capacitor at an MRI RF pulsed frequency orrange of frequencies is less than 0.1 ohms, wherein the MRI RF pulsedfrequency or range of frequencies comprises 64 MHz or 128 MHz. Adielectric loss tangent is measured in ohms at the MRI RF pulsedfrequency or range of frequencies that is less than five percent of thefilter capacitor's ESR. The filter capacitor is selected from the groupconsisting of a monolithic ceramic capacitor, a flat-through capacitor,a chip capacitor and an X2Y attenuator. A circuit board or substrate isdisposed on the device side, wherein the filter capacitor is mounted tothe circuit board or substrate and the circuit board or substrate islocated inside the conductive housing on the device side of the AIMD.The filter capacitor is the first filter capacitor connected to theconductive pathway disposed on the device side. In other words, thereare no other filters capacitors or electronic circuits containing filtercapacitors in the conductive pathway between and to the insulatorhermetically sealing the ferrule opening and the filter capacitor.

The circuit board or substrate may be disposed adjacent and attached toeither the ferrule and/or the insulator or the circuit board orsubstrate may be disposed distant from and unattached to either theferrule and/or the insulator.

The filter capacitor may be a first capacitor element of a multielementbroadband lowpass filter having at least one inductor, the multielementbroadband lowpass filter forming one of the group consisting of areverse L filter, an LL, a Pi and an n-element lowpass filter.

A diode or back-to-back diodes may be connected at a first diode end tothe conductive pathway and connected at a second diode end to either theferrule and/or the conductive housing. A back-to-back diode or diodesmay be connected in parallel with the filter capacitor to the conductivepathway and to either the ferrule and/or the conductive housing.

Another embodiment of an active implantable medical device systemincludes an implantable lead conductor having at least one electrodeconfigured to be connectable to biological cells or tissue and an activeimplantable medical device (AIMD). The AIMD includes a conductivehousing defining a body fluid side located outside the conductivehousing and defining a device side located inside the conductivehousing. An electrically conductive ferrule is hermetically sealed to ahousing opening in the conductive housing, the ferrule having a ferruleopening passing through the ferrule between the body fluid side and thedevice side. An insulator hermetically seals the ferrule opening. Aconductive pathway hermetically seals and is disposed through theinsulator between the body fluid side and the device side, where theconductive pathway in non-conductive relation with the ferrule, andwherein a distal end of the conductive pathway is detachably connectedto a proximal end of the implantable lead conductor. A filter capacitoris disposed within the conductive housing on the device side. The filtercapacitor includes: a capacitance of between 10 and 20,000 picofarads; adielectric body supporting at least two active electrode platesinterleaved with at least two ground electrode plates, wherein the atleast two active electrode plates are electrically connected to theconductive pathway on the device side, and the at least two groundelectrode plates are electrically coupled to either the ferrule and/orthe conductive housing; wherein the dielectric body comprises adielectric constant less than 1000; and wherein the filter capacitor isconfigured for EMI filtering of MRI high RF pulsed power by a lowequivalent series resistance (ESR), wherein the ESR is the sum of adielectric loss plus an ohmic loss, wherein the ESR of the filtercapacitor at an MRI RF pulsed frequency or range of frequencies is lessthan 2.0 ohms.

Another embodiment of an active implantable medical device (AIMD),includes: a conductive housing defining a body fluid side locatedoutside the conductive housing and defining a device side located insidethe conductive housing; an electrically conductive ferrule hermeticallysealed to a housing opening in the conductive housing, the ferrulehaving a ferrule opening passing through the ferrule between the bodyfluid side and the device side; an insulator hermetically sealing theferrule opening; a conductive pathway hermetically sealed and disposedthrough the insulator between the body fluid side and the device side,where the conductive pathway in non-conductive relation with theferrule, and the conductive pathway on the body fluid side connectableto an implantable lead conductor having at least one electrode; aprimary filter capacitor disposed within the conductive housing on thedevice side adjacent to the insulator, the primary filter capacitorbeing a feedthrough filter, wherein the primary filter capacitor is afirst filter capacitor connected to the conductive pathway disposed onthe device side; a secondary filter capacitor disposed within theconductive housing on the device side, wherein the secondary filtercapacitor is selected from the group consisting of a monolithic ceramiccapacitor, a flat-through capacitor, a chip capacitor and an X2Yattenuator. Both the primary and secondary filter capacitors eachinclude: a capacitance of between 10 and 20,000 picofarads; a dielectricbody supporting at least two active electrode plates interleaved with atleast two ground electrode plates, wherein the at least two activeelectrode plates are electrically connected to the conductive pathway onthe device side, and the at least two ground electrode plates areelectrically coupled to either the ferrule and/or the conductivehousing; wherein the dielectric body comprises a dielectric constantless than 1000; wherein the filter capacitors are configured for EMIfiltering of MRI high RF pulsed power by a low equivalent seriesresistance (ESR), wherein the ESR is the sum of a dielectric loss plusan ohmic loss, wherein the ESR of the filter capacitor at an MRI RFpulsed frequency or range of frequencies is less than 2.0 ohms. Acircuit board or substrate is located inside the conductive housing onthe device side of the AIMD, wherein the secondary filter capacitor ismounted to the circuit board or substrate.

An embodiment of a circuit board assembly includes: a circuit boardsubstrate configured to be installable inside a conductive housing of anactive implantable medical device (AIMD), where the AIMD would include:a conductive housing defining a body fluid side located outside theconductive housing and defining a device side located inside theconductive housing; an electrically conductive ferrule hermeticallysealed to a housing opening in the conductive housing, the ferrulehaving a ferrule opening passing through the ferrule between the bodyfluid side and the device side; an insulator hermetically sealing theferrule opening; and a conductive pathway hermetically sealed anddisposed through the insulator between the body fluid side and thedevice side, the conductive pathway in non-conductive relation with theferrule, and the conductive pathway on the body fluid side connectableto an implantable lead conductor having at least one electrode. Amultitude of electronic components are disposed onto the circuit boardsubstrate, the multitude of electronic components configured to controleither sensing and/or pacing pulses transmitted through the connectableimplanted lead conductor. A filter capacitor is disposed onto thecircuit board substrate. The filter capacitor includes: a capacitance ofbetween 10 and 20,000 picofarads; and a dielectric body supporting atleast two active electrode plates interleaved with at least two groundelectrode plates; wherein the at least two active electrode plates areconfigured to be electrically connectable to the conductive pathway onthe device side; wherein the at least two ground electrode plates areconfigured to be electrically connectable to either the ferrule and/orthe conductive housing; wherein the dielectric body comprises adielectric constant less than 1000; and wherein the filter capacitor isconfigured for EMI filtering of MRI high RF pulsed power by a lowequivalent series resistance (ESR), wherein the ESR is the sum of adielectric loss plus an ohmic loss, wherein the ESR of the filtercapacitor at an MRI RF pulsed frequency or range of frequencies is lessthan 2.0 ohms.

An embodiment of feedthrough assembly can be configured to beconnectable to a conductive housing of an active implantable medicaldevice (AIMD), the feedthrough assembly includes: an electricallyconductive ferrule configured to be hermetically sealable to a housingopening in the conductive housing, the ferrule having a ferrule openingpassing through the ferrule, wherein the ferrule defines a first sideopposite a second side about the ferrule opening; an insulatorhermetically sealing the ferrule opening; a conductive pathwayhermetically sealed and disposed through the insulator between the firstside and the second side, the conductive pathway in non-conductiverelation with the ferrule, wherein the conductive pathway on the firstside is configured to be connectable to an implantable lead conductorhaving at least one electrode; and a filter capacitor disposed on thesecond side of the ferrule. The filter capacitor includes: a capacitanceof between 10 and 20,000 picofarads; a dielectric body supporting atleast two active electrode plates interleaved with at least two groundelectrode plates, wherein the at least two active electrode plates areelectrically connected to the conductive pathway on the second side, andthe at least two ground electrode plates are electrically coupled to theferrule; wherein the dielectric body comprises a dielectric constantless than 1000; and wherein the filter capacitor is configured for EMIfiltering of MRI high RF pulsed power by a low equivalent seriesresistance (ESR), wherein the ESR is the sum of a dielectric loss plusan ohmic loss, wherein the ESR of the filter capacitor at an MRI RFpulsed frequency or range of frequencies is less than 2.0 ohms.

Other features and advantages of the present invention will becomeapparent from the following more detailed description, taken inconjunction with the accompanying drawings which illustrate, by way ofexample, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate the invention. In such drawings:

FIG. 1 is a wire-formed diagram of a generic human body showing a numberof exemplary implanted medical devices;

FIG. 2 is a pictorial view of an AIMD patient who is about to be placedinto an MRI scanner;

FIG. 3 shows a side view of the patient within the scanner showing anintense RF field impinging on the implanted medical device and itsassociated lead;

FIG. 4 is a top view of the patient in the MRI scanner showing onelocation of the AIMD and its associated lead;

FIG. 5 is a line drawing of a human heart with cardiac pacemaker dualchamber bipolar leads shown in the right ventricle and the right atrium;

FIG. 6 illustrates a dual chamber cardiac pacemaker with its associatedleads and electrodes implanted into a human heart;

FIG. 7 is a perspective view illustrating the rectangular feedthroughcapacitor mounted to a hermetic terminal;

FIG. 8 is an enlarged sectional view taken generally along the line 8-8of FIG. 7;

FIG. 9 is a perspective view of a round hermetic terminal showing a quadpolar RF diverter feedthrough capacitor;

FIG. 10 is an enlarged cross-sectional view taken generally along theline 10-10 from FIG. 9;

FIG. 11 is an electrical schematic diagram of the quadpolar feedthroughcapacitor of FIGS. 7-10;

FIG. 12 is a sectional view of an embodiment of a novel hermeticterminal subassembly installed in a housing of an AIMD;

FIG. 13 is a sectional view of another embodiment of a hermetic terminalsubassembly now showing a capacitor with a filled and a bore-coated via;

FIG. 14 is a perspective view of a monolithic ceramic capacitor (MLCC);

FIG. 15 is a cross-sectional view of the monolithic ceramic capacitor,taken along the line 15-15 of FIG. 14;

FIG. 16 is an electrical schematic diagram of an ideal MLCC capacitor asillustrated in FIGS. 14 and 15;

FIG. 17 is a flat-through three-terminal capacitor;

FIG. 18 illustrates the internal electrode plates of the flat-throughcapacitor of FIG. 17;

FIG. 18A is the electrical schematic of FIGS. 17 and 18;

FIG. 19 is a perspective exploded view of a multi-lead hermeticfeedthrough with substrate mounted MLCCs showing use of a substratebetween the feedthrough and the filter support assembly;

FIG. 19A is the electrical schematic of FIG. 19;

FIG. 20 illustrates a cross-sectional view of an MLCC capacitor mountedto separate circuit traces;

FIG. 21 is a schematic representation explaining the elements that arecomponents of the FIG. 20 capacitor's equivalent series resistance(ESR);

FIG. 22 is an equation that relates the capacitance with the capacitor'sactive area, dielectric constant, number of electrode plates anddielectric thickness;

FIG. 23 shows the difference between an ideal capacitor and a realcapacitor, including dielectric loss tangent and dissipation factor;

FIG. 24 gives the formulas for capacitive reactance, dissipation factor,equivalent series resistance (ESR) and dielectric loss tangent;

FIG. 25 is an equivalent circuit model for a real capacitor;

FIG. 26 is a schematic illustrating a simplified model for capacitorESR;

FIG. 27 is a graph illustrating capacitor dielectric loss versusfrequency;

FIG. 28 is a graph illustrating normalized curves which show thecapacitor equivalent series resistance (ESR) on the y axis, versusfrequency on the x axis;

FIG. 29 illustrates the reactance and real losses of a 2000 picofaradX7R feedthrough capacitor;

FIG. 30 illustrates the reactance and real losses of a 2000 picofaradCOG (NPO) capacitor;

FIG. 31 is a graph illustrating capacitor equivalent series resistanceversus frequency as illustrated in a sweep from an Agilent E4991Amaterials analyzer;

FIG. 32 is a cross-sectional view of a lower k MLCC with an increasednumber of electrode plates to minimize ESR;

FIG. 33 is an equation showing that the total high frequency electroderesistive losses drop in accordance with the parallel plate formula forcapacitor electrodes;

FIG. 34 is a cross-sectional view of a quad polar feedthrough capacitorsimilar to FIGS. 9 and 10 except that it is low ESR and designed formaximal heat flow;

FIG. 35 is a partial section taken from section 35-35 from FIG. 34illustrating dual electrode plates to minimize capacitor ESR andmaximize heat flow out of the capacitor;

FIG. 36 is similar to FIG. 35 except that just the ground electrodeplates have been doubled;

FIG. 37 illustrates a family of lowpass filters indicating the presentinvention can be anything from a simple diverter capacitor 140 to an “n”element lowpass filter;

FIG. 38 illustrates a feedthrough diverter capacitor, a bandstop filterand an L-C trap;

FIG. 39 illustrates a cardiac pacemaker with a diverter feedthroughcapacitor and also a circuit board mounted chip capacitor filter whichforms a composite filter and also spreads out heat generation;

FIG. 39A illustrates the electrical schematic of FIG. 39;

FIG. 39B is a sectional view now showing the capacitor mounted to aflexible connection;

FIG. 39C illustrates the electrical schematic of FIG. 39B;

FIG. 40 is a fragmented perspective view of an EMI shield conduitmounted to a circuit board having multiple MLCC chip capacitors;

FIG. 41 is a cross-sectional view of an improved flex cable embodyingthe present invention;

FIG. 42 is a sectional view taken along line 42-42 of FIG. 41;

FIG. 43 is a sectional view taken along the line 43-43 of FIG. 41,illustrating an alternative to the internal circuit traces describedwith respect to FIG. 42;

FIG. 44 is a sectional view taken along line 44-44 of FIG. 41,illustrating one of a pair of coaxially surrounding shields disposedabout the circuit trace;

FIG. 45 is a perspective view of the flex cable of FIG. 41 connected toa circuit board or substrate having a flat-through capacitor;

FIG. 46 is the top view of the flat-through capacitor from FIG. 45;

FIG. 47 illustrates the active electrode plates of the flat-throughcapacitor of FIGS. 45 and 46;

FIG. 48 illustrates the ground electrode plate set of the flat-throughcapacitor of FIGS. 45 and 46;

FIG. 49 illustrates a family of low pass filters, which is very similarto the family of low pass filters described in FIG. 37;

FIG. 49A is similar to FIG. 49 now showing the attenuation curve for afeedthrough capacitor with chip capacitor;

FIG. 50 illustrates that the high energy dissipating low ESR capacitorcan be used in combination with other circuits;

FIG. 51 shows a perspective view of an MLCC capacitor that is similar inits exterior appearance to the prior art MLCC capacitor previouslydescribed in FIGS. 14 and 15;

FIG. 52 is a sectional view taken along lines 52-52 of FIG. 51;

FIG. 53 is the electrical schematic representation of FIGS. 51 and 52;

FIG. 54 is a bipolar hermetic seal having a ferrule and two leadspassing through the conductive ferrule in insulative relationship;

FIG. 55 is the electrical schematic representation of FIG. 54;

FIG. 56 is similar to FIG. 6 showing a breakaway cross-section of atypical AIMD with novel capacitors mounted to an internally disposedcircuit board;

FIG. 56A is the electrical schematic of FIG. 56;

FIG. 57 is very similar to FIG. 56 except that a diode array has beenadded;

FIG. 58 is the electrical schematic representation of FIG. 57;

FIG. 59 is very similar to FIG. 58 except the high voltage protectiondiode array is shown on the other side of the low ESR capacitors;

FIG. 60 is an electrical schematic of a back-to-back diode placed inseries taken from lines 60-60 of FIG. 59;

FIG. 61 is very similar to FIG. 56 except that the RF grounding straphas been replaced with a simple leadwire connection;

FIG. 62 is very similar to FIG. 61 now with the grounding leadwirerouted directly to the ferrule of the hermetic terminal subassembly;

FIG. 63 is very similar to prior art FIG. 17 that illustrated aflat-through type of feedthrough capacitor;

FIG. 64 is very similar to prior art FIG. 18 that illustrated aflat-through type of feedthrough capacitor;

FIG. 65 is the electrical schematic representation of FIGS. 63-64;

FIG. 66 is similar in outline to the flat-through capacitor of FIG. 17;

FIG. 67 shows the internal active electrode plates of FIG. 66 now withthe dielectric removed;

FIG. 68 shows the internal ground electrode plates of FIG. 66 now withthe dielectric removed;

FIG. 69 shows how the active and ground electrode plates of FIG. 66 nestparallel to one another with the dielectric removed; and

FIG. 70 shows the electrical schematic representation of FIGS. 66-69.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates various types of active implantable medical devicesreferred to generally by the reference numeral 100 that are currently inuse. FIG. 1 is a wire formed diagram of a generic human body showing anumber of exemplary implanted medical devices. Numerical designation100A is a family of implantable hearing devices which can include thegroup of cochlear implants, piezoelectric sound bridge transducers andthe like. Numerical designation 100B includes an entire variety ofneurostimulators and brain stimulators. Neurostimulators are used tostimulate the Vagus nerve, for example, to treat epilepsy, obesity anddepression. Brain stimulators are similar to a pacemaker-like device andinclude electrodes implanted deep into the brain for sensing the onsetof the seizure and also providing electrical stimulation to brain tissueto prevent the seizure from actually happening. Numerical designation100C shows a cardiac pacemaker which is well-known in the art. Numericaldesignation 100D includes the family of left ventricular assist devices(LVAD's), and artificial hearts, including the recently introducedartificial heart known as the Abiocor. Numerical designation 100Eincludes an entire family of drug pumps which can be used for dispensingof insulin, chemotherapy drugs, pain medications and the like. Insulinpumps are evolving from passive devices to ones that have sensors andclosed loop systems. That is, real time monitoring of blood sugar levelswill occur. These devices tend to be more sensitive to EMI than passivepumps that have no sense circuitry or externally implanted leadwires.100F includes a variety of implantable bone growth stimulators for rapidhealing of fractures. Numerical designation 100G includes urinaryincontinence devices. Numerical designation 100H includes the family ofpain relief spinal cord stimulators and anti-tremor stimulators.Numerical designation 100H also includes an entire family of other typesof neurostimulators used to block pain. Numerical designation 100Iincludes a family of implantable cardioverter defibrillator (ICD)devices and also includes the family of congestive heart failure devices(CHF). This is also known in the art as cardio resynchronization therapydevices, otherwise known as CRT devices. Numerical designation 100Jillustrates an externally worn pack. This pack could be an externalinsulin pump, an external drug pump, an external neurostimulator or evena ventricular assist device. 100K illustrates the insertion of anexternal probe or catheter. These probes can be inserted into thefemoral artery, for example, or in any other number of locations in thehuman body.

Referring to U.S. 2003/0050557, Paragraphs 79 through 82, the contentsof which are incorporated herein, metallic structures, particularlyleads, are described that when placed in MRI scanners, can pick up highelectrical fields which results in local tissue heating. This heatingtends to be most concentrated at the ends of the electrical structure(either at the proximal or distal lead ends). This safety issue can beaddressed using the disclosed systems and methods of the presentinvention. A significant concern is that the distal electrodes, whichare in contact with body tissue, can cause local tissue burns.

As defined herein, an active implantable medical device (AIMD) includesany device or system that is designed to be implanted within a humanbody either totally or partially and includes at least one electroniccircuitry. AIMDs may have primary or secondary batteries as their energysources. AIMDs may also harvest energy from the body either throughmechanical motion or through chemical or through biochemical batterycell type effects. An AIMD may also contain a resonant circuit wherebyit captures energy from external pulsing electromagnetic field. Anexample of this would be what is known in the industry as the Bion©. Ingeneral, AIMDs are connected to either a leadwire or are directlyconnected to electrodes without a leadwire wherein, these electrodes arecontactable to biological cells. AIMD electrodes may be used for therapydelivery, sensing of biological signals or both. AIMDs may also beintegrated with fiber optic cables and receive their power or signalsoptically wherein there is an optical converter which may convert theoptical signals to either digital signals or to power.

A subclass of AIMDs is known as cardiac implantable electronic devices(CIEDs). CIEDs include all types of pacemakers, implantable cardioverterdefibrillators, implantable loop recorders, subcutaneous ICDs and thelike. Another subclass of AIMDs includes all types of neurostimulators,including, but not limited to spinal cord stimulators, deep brainstimulators, urinary incontinence stimulators and the like. An AIMD mayinclude an external component, such as an RF transmitter, an RFtelemetry device or even a worn wrist watch, which sends signals to animplanted device and its associated electrodes. In other words, theAIMD, as defined herein can have externally worn components in additionto implanted components.

In general, an AIMD usually has a housing which hermetically shields andprotects one or more internal electronic circuits. As defined herein,the AIMD has a body fluid side, which is defined as anything on theexterior of the AIMD housing. It also has a device side. The deviceside, which can also be known as an inboard side, refers to the interiorof the AIMD housing and any components or electronic circuits that maybe disclosed within it.

As used herein, the lead means an implanted lead, including itsconductors and electrodes that have electrodes that are in contact withbody tissue. In general, for an AIMD, the term lead means the lead thatis outside of the AIMD housing and is implanted or directed into bodytissues. The term leadwire as used herein refers to the wiring orcircuit traces that are generally inside of the active implantablemedical device (AIMD) and are not exposed directly to body fluids.

FIG. 2 illustrates an AIMD patient 102 about to be conveyored into anMRI machine 104. Imaging processing equipment is shown as 106.

FIG. 3 is a side view showing the patient 102 within the MRI scannerbore 104. Intense RF field 108 is generated by the scanners bird cagecoil. As can be seen, this RF field is impinging on both the implantedcardiac pacemaker 100C and its associated leads 110.

FIG. 4 is a top view of the patient 102 inside the MRI scanner bore 104.As can be seen, the pacemaker 100C is in a left pectoral pocket with theleads 110 routed transvenously down into the interior chambers of theheart.

FIG. 5 is a line drawing of a human heart 112 with cardiac pacemaker100C dual chamber bipolar leads shown in the right ventricle 114 and theright atrium 116 of a human heart 112.

Referring once again to FIG. 5, as previously mentioned, it is veryimportant that this lead system does not overheat during MRI proceduresparticularly at or near the distal tip 118 a, 118 b electrodes and ringelectrodes 120 a, 120 b. If either or both the distal tip 118 a, 118 band ring 120 a, 120 b electrode become overgrown by body tissue,excessive overheating can cause scarring, burning or necrosis of saidtissues. This can result in loss of capture (loss pacing pulses) whichcan be life-threatening for a pacemaker dependent patient. FIG. 6 is apectoral view of a prior art cardiac pacemaker 100C showing dual chamberbipolar leads 110, 110′ routed to distal tip electrodes 118 a and 118 band distal ring electrode 120 a and 120 b. As can be seen, the leads110, 110′ are exposed to a powerful RF-pulsed field from an MRI machine.This induces electromagnetic energy on the leads which are coupled viaISO Standard IS-1 or DF-1 connectors 126, 128 through header block 138which connects the leads to electronic circuits 130 inside of thehermetically sealed pacemaker housing 124. A hermetic seal assembly 132is shown with a metal ferrule 134 which is generally laser welded intothe titanium housing 124 of the cardiac pacemaker 100C. Lead wires 136 athrough 136 d penetrate the ferrule 134 of the hermetic seal innon-conductive relation. Glass seals or gold brazed alumina insulatorsare formed to perfect the hermetic seal which keeps body fluids fromgetting to the inside of the pacemaker housing 124.

FIGS. 7-8 illustrate a prior art rectangular quadpolar feedthroughcapacitor (planar array) 140 mounted to the hermetic terminal 132 of acardiac pacemaker in accordance with U.S. Pat. No. 5,333,095 toStevenson et al. the contents of which are incorporated herein.Referring back to FIG. 6, one can see that the capacitor 140 is mountedadjacent to the ferrule 132 or the insulator 156. As illustrated inFIGS. 7-8, in a typical broadband or lowpass EMI filter construction, aceramic feedthrough filter capacitor 140 is used in a hermeticfeedthrough assembly 132 to suppress and decouple undesired interferenceor noise transmission along one or more terminal pins 142, and maycomprise a capacitor having two sets of electrode plates 144 and 146embedded in spaced relation within an insulative dielectric substrate orbase 148, formed typically as a ceramic monolithic structure. One set ofthe electrode plates 144 is electrically connected at an inner diametercylindrical surface to metallization 166 of the capacitor structure 140to the conductive terminal pins 142 utilized to pass the desiredelectrical signal or signals (see FIG. 11). The other or second set ofelectrode plates 146 is coupled to metallization 164 at a sidewall ofthe dielectric providing an outer edge surface of the capacitor 140through metallization to a rectangular ferrule 134 of conductivematerial. In the prior art, without regard to high frequency capacitorESR, the number and dielectric thickness spacing of the electrode platesets 144, 146 varies in accordance with the capacitance value and thevoltage rating of the capacitor 140.

In operation, the coaxial capacitor 140 permits passage of relativelylow frequency electrical signals along the terminal pins 142, whileshielding and decoupling/attenuating undesired interference signals oftypically high frequency to the conductive housing 124. Feedthroughcapacitors 140 of this general type are available in unipolar (one),bipolar (two), tripolar (three), quadpolar (four), pentapolar (five),hexpolar (6) and additional lead configurations. Feedthrough capacitors140 (in both discoidal and rectangular configurations) of this generaltype are commonly employed in implantable cardiac pacemakers anddefibrillators and the like, wherein the pacemaker housing isconstructed from a biocompatible metal such as titanium alloy, which iselectrically and mechanically coupled to the hermetic terminal pinassembly which is in turn electrically coupled to the coaxialfeedthrough filter capacitor 140. As a result, the filter capacitor andterminal pin assembly prevent entrance of interference signals to theinterior of the pacemaker housing 124, wherein such interference signalscould otherwise adversely affect the desired cardiac pacing ordefibrillation function.

As one can see in FIG. 7, the conductive polyimide material 152 connectsbetween the capacitor metallization 164 and the gold braze area 154. Thegold braze 154 forms a metallurgical bond with the titanium andprecludes any possibility of an unstable oxide forming. Gold is a noblemetal that does not oxidize and remains very stable even at elevatedtemperatures. The novel construction methodology illustrated in FIG. 7guarantees that the capacitor ohmic losses will remain very small at allfrequencies. By connecting the capacitor's electrode plates to a lowresistivity surface such as gold, one is guaranteed that this connectionwill not substantially contribute to the capacitor's overall ESR.Keeping the ESR as low as possible is very important for diverting ahigh amount of RF current such as that induced in the lead system by MRIscanners. One is referred to U.S. Pat. No. 6,765,779 to Stevenson etal., for additional information on electrically connecting tonon-oxidized surfaces, the contents of which are incorporated herein byreference.

FIG. 8 is a cross-section of the capacitor shown in FIG. 7. One can seethat the gold braze (or weld) areas 154 a and 154 b that form thehermetic seal between the alumina insulator 156 and the titanium ferrule134 are desirably on the feedthrough capacitor side. This makes it easyto manufacture the gold bond pad area 158 for convenient attachment ofthe conductive thermal-setting material 152. In other words, by havingthe gold braze hermetic seals 154 on the same side as the gold bond padarea 158, these can be co-formed in one manufacturing operation in agold braze vacuum furnace. Further, a unique insulative material 160 isdisposed between the capacitor 140 and the underlying hermetic terminal132.

FIG. 9 is a quad polar feedthrough capacitor 140 mounted to a hermeticterminal 132 similar to that described in FIG. 7 except that in thiscase, the structure is round or discoidal.

FIG. 10 is a cross-sectional view taken generally from section 10-10from FIG. 9. There are four feedthrough leadwires 142 which extendthrough the capacitor 140, which has a ground electrode plate set 146and an active electrode plate set 144.

As shown there are only two active electrodes 144 and three groundelectrodes 146. This low electrode plate count results in a feedthroughdiverter capacitor 140 that has a relatively high ESR at highfrequencies. In a recent experiment conducted by the inventors, atypical EIA X7R 400 picofarad feedthrough capacitor with only fourelectrode plates had an ESR at 64 MHz of 4.8 Ohms. Re-design of the samegeometry (size) capacitor with an EIA NPO dielectric for a 400 picofaradcapacitor with over 20 electrodes resulted in an ESR at 64 MHz ofapproximately 300 milliohms (0.3 Ohms). This sixteen to one reduction in64 MHz is a dramatic illustration of the importance of designing theAIMD MRI diverter capacitor 140 for low ESR. For example, for an X7Rcapacitor the impedance would be the square root of the sum of thecapacitor's reactance squared plus the ESR squared. A 400 pF capacitorhas a reactance of 2.49 Ohms at 64 MHz. This results in a capacitorimpedance Z which is equal to −j2.49+4.8 or approximately 5.41 Ohms.Assuming an MRI induced RF voltage at the AIMD input at 64 MHz of 10Volts, the RF current diverted through the X7R capacitor is 10 Voltsdivided by 5.41 Ohms which is 1.85 Amps. The power dissipation due tothe X7R capacitor's ESR (I²R) is (1.85)²(4.8)=16.43 Watts. This amountof power dissipation is very excessive for such a small component andwill cause a temperature rise of over 20 degrees C. On the other hand, a400 pF NPO capacitor's impedance is equal to −j2.49−0.3 or Z=2.51 Ohms.This lower impedance will result in a much better filter (higherattenuation) and will drop the RF voltage from 10 Volts to approximately3.71 Volts. This voltage drop is caused by the lead's characteristicimpedance and the fact that more current has been drawn through thisimpedance. This causes a voltage drop in the lead's characteristicimpedance as measured at the input to the AIMD. The RF current throughthe NPO capacitor is then 3.71 Volts divided by Z of 2.51 Ohms which is1.48-amps. The power dissipation (I²R) is (1.48 Amps)²(0.3 Ohm) whichequals 0.66 Watts which will result in a much smaller temperature rise.Accordingly, the low ESR diverter capacitor 210, 410 design of thepresent invention offers the following advantages: (1) it has a muchlower impedance at 64 MHz and is therefore a more effective EMI filter,and; (2) because it offers higher attenuation, it therefore acts toreduce the MRI induced RF voltage at the input to the AIMD; and (3) aswill be shown in the present invention, the diverter capacitor 210, 410can be designed to conduct or convect heat away and dissipate it over alarger surface area. (Note that the above example corrects the parentapplication's typographical and numerical errors.)

The above examples of ESR and impedance are just illustrated examples ofmany thousands of possibilities. For active implantable medical devices,in general, capacitance values range anywhere from 300 pF to 15,000 pF.Each design has a different physical geometry in size and internalelectrode plates. In other words, there are many other examples thatwould have different values of ESR. However, the general principlesillustrated above do apply across the board. Lower k dielectrics willalways mean a higher number of electrode plates and hence, a lower ESR.That means that the low ESR designs will have much less heating of thecapacitor itself in an MRI environment.

The capacitor 140 is bonded with an insulating washer 162 to thehermetic terminal 132. An electrical attachment 152 is made using athermal-setting conductive adhesive between the feedthrough capacitoroutside diameter metallization 164 and gold braze surface 158. Thenecessity to make an oxide free attachment between the feedthroughcapacitor 140 and the ferrule 134 is described in U.S. Pat. No.6,765,779. An insulator 156 such as glass compression, glass fusion sealor alumina ceramic is hermetically sealed to the ferrule 134 by means ofgold braze 154 a. The four leadwires 142 are also hermetically sealed tothe insulator 156 via gold braze rings 154 b (there are four of these).The feedthrough capacitor active electrode plates 144 are attached byco-firing to the capacitor feedthrough hole inside diametermetallization 166. Conductive electrical material 168 is used to attachthe metallization 166 to each one of the leadwires 142.

Referring once again to FIG. 8, one can see that there are only twoactive electrode plates 144 and two ground electrode plates 146. A lowelectrode plate count is typically the case for prior art filteredfeedthrough (diverter) capacitors 140 used in AIMD applications such ascardiac pacemakers, ICDs and the like. Another reason that thecapacitance value is generally low is that a high capacitance valuewould load down the output of the AIMD. For example, too high of acapacitance value would distort pacemaker therapeutic pulses and alsorob energy from the system. An even more extreme example would be thecase of an implantable cardioverter defibrillator, wherein too high of afilter capacitance value would seriously degrade the high voltagemonophasic or biphasic shock wave form. In the experience of theinventors, the capacitance value for AIMD diverter capacitor 140 is in arelatively narrow range from 10 to 20,000 picofarads. In most cases, thecapacitance value is between 350 and 10,000 picofarads. Having acapacitance value between 350 and 10,000 picofarads effectivelyattenuates most emitters from which AIMDs can be affected. This includesmicrowave ovens, cellular telephones and the like, which typicallyoperate in the GHz frequency range. The thickness 170 of the capacitorhowever, cannot be below a certain minimum or the barium titanate basedceramic capacitor 148 will become too fragile. The entire hermeticterminal 132 and the feedthrough capacitor 140 must be able to withstandthermal cycles and shocks including installation by laser welding 157into the AIMD housing 124. Accordingly, it is very unusual to see adiverter capacitor 140 thickness 170 of less than 20/1000 of an inch(0.020 inches or 20 mils). Correspondingly, when one looks at a typicalprior art feedthrough capacitor 140 for human implant in cross-section,one sees that there are very few electrodes 144, 146 relative to itsoverall thickness 170. In fact, there are usually a number of blankdielectric (no electrodes) cover sheets/layers 172 added on the topand/or bottom of the capacitor 140 consisting of ceramic material whichis co-fired to add mechanical strength. However, there is a seriousdownside to having very few electrode plates 144, 146, and that is thatthe high frequency equivalent series resistance (ESR) of the capacitorincreases. For prior art AIMD filter or diverter capacitor 140 havingsignificant dielectric and/or ohmic resistance at high frequenciessimply does not matter. This is because the power induced from a typicalemitter, such as a cellular telephone or microwave oven results in atrivial amount of RF current flowing through diverter capacitor 140.Even in the most extreme examples, only a few milliwatts of heat wouldbe generated within the capacitor structure itself. However, for highpower RF current handling applications, such as diverting MRI induced RFenergy, the capacitor dielectric loss and high frequency ESR becomecritical and must be kept as low as possible. Accordingly, it is afeature of the present invention to have a relatively high number ofelectrode plates 144, 146 (generally greater than 10). However, with ahigh k barium titanate based ceramic dielectric with a dielectricconstant of around 2500, a high number of electrode plates would resultin a very high (too high) capacitance value. A way to solve this is touse a relatively low dielectric constant material, such as EIA StandardNPO material. NPO material has a much lower k (generally, in the area of60 to 90). Accordingly, in order to achieve the desired capacitancevalue (in the range of 350 to 10,000 picofarads), a much greater numberof electrode plates is required. The higher number of electrode platescreates more parallel paths for RF current flow and greatly reduces theESR of the feedthrough capacitor. One is referred to the equationillustrated in FIG. 22 to explain the relationship between capacitanceand the number of electrode plates and other factors.

FIG. 11 is a schematic diagram of the quad polar feedthrough capacitor140 of FIGS. 7-10. Feedthrough capacitors are three-terminal deviceslabeled in FIG. 1 as 141 a, 141 b, and 141 c.

FIG. 12 is a sectional view of an embodiment of a novel hermeticterminal subassembly installed in a housing of an AIMD taken from U.S.Pat. No. 8,653,384 as FIG. 17, the contents of which are incorporatedherein with this reference. The outside diameter of the alumina hermeticinsulator 156 has metalized surfaces, which are adhesion and wettingsurfaces so that gold braze 154 can be melted and hermetically bonded tothe alumina hermetic insulator 156 and the ferrule 134 of the hermeticterminal assembly. The ferrule 134 may be installed into the AIMDhousing 124 by laser welding 157, or the like. In this embodiment it isa goal to eliminate the highly expensive biocompatible and nobleleadwires 142, as previously illustrated. Instead of a feedthroughleadwire 142, this embodiment comprises a pure platinum filled via hole142′. It is a novel feature of this embodiment that this via holematerial 142′ be of essentially pure platinum that is co-fired with theessentially high purity alumina ceramic substrate 156.

In general, AIMD hermetic seals have a body fluid side and a deviceside. In general, the device side (inboard side) is located inside theconductive housing of the AIMD. For example, for a hermetic seal itwould have conductive passages or leadwires passing through it. Theconductive passages or leadwires would be exposed to body fluid on thebody fluid side and on the inboard side or device side, they would belocated inside of the AIMD housing and the leads or conductive pathwayswould be connectable to AIMD internal electronic circuits. As can beseen, the conductive pathway through the insulator 156 between the bodyfluid side and the device side can be made from a conductive wire 142 ornow as a conductive paste 142′. The conductive pathway between the bodyfluid side and the device side can also be a combination of conductiveinserts into conductive pastes and the like.

FIG. 13 is a sectional view of another embodiment of a hermetic terminalsubassembly now showing a capacitor with a filled and a bore-coated viaalso taken from U.S. Pat. No. 8,653,384 as FIG. 22. One is directed toU.S. Pat. No. 8,179,658, which is incorporated herein by this reference,which shows a capacitor via within internal metallization 166electrically connected to a solid feedthrough leadwire. In the presentembodiment, the feedthrough capacitor 140 has been mounted directly tothe surface of the co-fired high purity alumina hermetic terminalsubassembly with one or more pure platinum filled vias 142 a′ and 142b′. The feedthrough capacitor 140 is first placed on the bottom surfaceof the co-fired high purity alumina hermetic terminal subassembly withone or more pure platinum (or equivalent) filled vias. In thisembodiment, an adhesively backed insulator washer 153 is used to affixthe feedthrough capacitor 140 onto the surface of the alumina substrate156.

There are two different methods of electrical attachment to thefeedthrough capacitor illustrated. In the left hand hole, we have asolid fill of a solder, braze or thermal-setting conductive material155. A simplified electrical attachment is shown on the right sidewherein, a solder bump or ball grid array (BGA) 151 is first dispensedand then the capacitor is aligned and placed over it. Then, temperatureis applied to reflow the solder into place as shown. The solder makeselectrical contact with the platinum filled via hole 142 a′, 142 b′ andalso with the capacitor terminations 166.

In accordance with good EMC principles, the feedthrough capacitor 140 isdisposed immediately at the point of EMI ingress into the inside of thedevice housing 124. In this way, high frequency EMI can be decoupled anddiverted to the device housing 124 without adversely effecting AIMDsensitive electronic circuits. Feedthrough capacitor active electrodeplate sets 144 a and 144 b are both connected to the capacitor insidediameter metallization 166. The capacitor ground electrode plate sets146 make contact with the capacitor outside diameter or perimetermetallization 164. An electrical connection 152 is made from thecapacitor outside diameter ground metallization 164 and the gold braze154 of ferrule 134. This makes a low impedance oxide free electricalconnection which is superior for high frequency performance.

FIG. 14 is a prior art multi layered ceramic capacitor (MLCC) 140′.These are made by the hundreds of millions per day to service consumerelectronics and other markets. Virtually all computers, cell phones andother types of electronic devices have many of these. One can see thatthe MLCC 140′ has a body generally consisting of a high dielectricconstant ceramic 148′ such as barium titanate. It also has a pair ofsolderable termination surfaces 164 a, 164 b at either end. Thesesolderable termination surfaces 164 a, 164 b provide a convenient way tomake a connection to the internal electrode plates 144, 146 of the MLCCcapacitor 140′. FIG. 14 can also take the shape and characteristics of anumber of other types of capacitor technologies, including rectangular,cylindrical, round, tantalum, aluminum electrolytic, stacked film or anyother type of capacitor technology.

FIG. 15 is a sectional view taken from section 15-15 in FIG. 14. TheMLCC 140′ includes a left hand electrode plate set 144 and a right handelectrode plate set 146. One can see that the left hand electrode plateset 146 is electrically connected to the external metallization surface164 a. The opposite, right hand electrode plate set 146 is shownconnected to the external metallization surface 164 b. Prior art MLCC140′ and equivalent chip capacitors are also known as two-terminalcapacitors. That is, there are only two ways electrical energy canconnect to the body of the capacitor. In FIGS. 14 and 15, the firstterminal 174 is on the left side and the second terminal 176 is on theright side. As defined herein, MLCC capacitors are two-terminal devices.In contrast, feedthrough capacitors are three-terminal devices whichhave very low self-inductance and make excellent high frequency EMIfilters.

FIG. 16 is the schematic diagram of the MLCC chip capacitor 140′illustrated in FIGS. 14 and 15.

FIG. 17 illustrates another type of prior art 3-terminal filtercapacitor known as a flat-through capacitor 140″. It is connected ateach end to a circuit trace 178 a, 178 b. A circuit current 180 passesall the way through the capacitor 140″. The capacitor 140″ is alsoconnected to ground circuit paths 182 a, 182 b. The overlap of theactive electrodes and the ground electrodes creates the capacitance.

FIG. 18 illustrates the internal electrode plates of the flat-throughcapacitor 140″ of FIG. 17. A set of ground plates is illustrated as 146.The through electrode plate 144 is connected to capacitor terminationsurfaces 164 a, 164 b.

FIG. 18A is the schematic of the 3-terminal flat-through capacitor ofFIG. 17. One can see that it is a true 3-terminal device consisting ofterminals 164 a, 164 b and ground, which is the AIMD housing 124. Asshown, the circuit current passes all the way through the capacitor, asillustrated.

FIG. 19 illustrates a method of attaching MLCC chip capacitors 140′directly to the hermetic terminal 132. In accordance with the presentinvention, the MLCC capacitors 140′ would be of relatively lowdielectric constant, like NPO such that they will have a high number ofelectrode plates thereby minimizing their ESR. This would make them veryeffective in diverting high levels of RF current at an MRI RF pulsedfrequency. One is referred to U.S. Pat. Nos. 5,896,267 and 5,650,759,both to Hittman et al., which more thoroughly describe the use of MLCCcapacitors as filters attached at or near the hermetic terminal of anactive implantable medical device. These two patents are incorporatedherein by reference.

FIG. 19A is the schematic diagram of the bipolar MLCC filter of FIG. 19.As shown, these are 2-terminal capacitors with 1 terminal connected tothe leadwire 142 and the other terminal connected to ground, which isalso the AIMD housing 124.

FIG. 20 is a cross-section of a typical MLCC capacitor 140′, such asthose used in FIGS. 14 and 19 (except that the ESR would be high due tothe low number of electrodes 144,146). The principles of thiscross-section are also equally applicable to any type of feedthroughcapacitor 140, such as that described in FIGS. 7 and 9. In general, theequivalent series resistance of a capacitor depends upon a number ofvery important variables. The capacitor's ESR is the sum of theconnection resistance (R_(e)) 184, the resistance of attachmentmaterials (R_(a)) 186, the resistance of capacitor metallization (usedto attach to internal electrode plates) (R_(m)) 188, the resistance ofthe electrodes (R_(e)) 190 and 190′ and also the resistance of thedielectric loss tangent (R_(DL)) 192. There is also another type ofresistance (not shown) which occurs at very high frequency, known asskin effect (R_(s)). This is a situation in which the bulk of thecurrent flow is on the skin of electrodes and circuit connectionsinstead of uniformly distributed throughout a conductor. This has theaffect also of increasing a capacitor's ESR. In general, for typical MRIRF pulsed frequencies, skin effect can be ignored (it's mostly a greaterthan 500 MHz phenomenon).

FIG. 21 is the schematic diagram from FIG. 20 showing that for thesepurposes, the capacitor's ESR is the sum of the connection resistance(R_(c)) 184, the connection material (R_(a)) 186, the metallization(R_(m)) 188, the electrode plate resistance (R_(e)) 190 and thecapacitor's dielectric loss (R_(DL)) 192. The capacitor's dielectricloss (R_(DL)) 192 is frequency variable, which will be explained infurther detail. For a well designed and properly installed capacitor,many of these resistances are so small that they can be ignored. Forexample, referring once again to FIG. 20, if the capacitor metallization(R_(m)) 188 is well designed and properly attached, it will have atrivially small resistance. In a similar fashion, if the electricalattachment material (R_(a)) 186 is a solder or a proper thermal-settingconductive adhesive, it will also have a trivial amount of resistance.If the system is attached to gold or another similar non-oxidizedsurface, then the connection resistance (R_(c)) 184 would also betrivially small or about zero. Referring once again to FIG. 21, one cansee that the total ohmic losses are R_(o) 200, and in this case, R_(o)consists almost entirely of the total electrode plate resistance(R_(e)(total)) 190. This is why it is so important in the presentinvention to maximize the number of electrode plates. At high frequency,the ohmic loss of the low dielectric constant capacitor is almostentirely due to the resistive loss of the active and ground electrodeplates (R_(e)(total) 190).

FIG. 22 gives the equation relating capacitance to the dielectricconstant k, the active (overlap area) of the electrode plates A, thenumber of electrode plates n and the dielectric thickness d. Since thedielectric constant k is directly related to the capacitance C, one cansee how dramatically the capacitance would rise when the dielectricconstant k is 2500 as opposed to a k below 200 for an EIA Class Idielectric. Assuming a constant dielectric thickness d for a particularvoltage rating, the only way to increase the capacitance to its originalvalue, would be to greatly increase the number of electrode plates. Inthe prior art, this would be counterintuitive. However, in the presentinvention, this is exactly what we want to do. A high number ofelectrode plates drives down the high frequency ohmic losses and therebygreatly increases the efficiency of the capacitor to pull RF energy outof an implanted lead during MRI scans. In addition, the high number ofelectrode plates has a very low equivalent series resistance at the MRIRF-pulsed frequency, thereby significantly reducing the amount of heatthat would be produced in the filter diverter capacitor 140.

FIG. 23 illustrates an ideal capacitor 194 and also a non-idealcapacitor 196 which consists of an ideal capacitor 194 in series withits ESR 198. For the purposes of the present discussion, a capacitor'sseries inductance or insulation resistance (a parallel resistance) canboth be ignored. This is because the inductance of feedthroughcapacitors is quite low at MRI RF-pulsed frequencies. Further, thecapacitor's insulation resistance is generally in the megohms or gigohmsrange, which is so high, it can also be ignored as a parallel path. Alsoshown is a graph of the impedance plane showing the capacitor ESR in thereal axis and the capacitive reactance −jX_(c) shown on the imaginaryaxis. The capacitor's loss tangent 6 is also illustrated.

In FIG. 24, equations are given for capacitive reactance X_(c),impedance Z, and dissipation factor DF and also for the tangent of δwhich is also defined as dissipation factor DF. Historically,dissipation factor has been expressed as a percent, such as 2.5%maximum. This would mean that the allowable dissipation factor would be2.5% of the capacitor's capacitance reactance at a particular frequency.Usually, due to dielectric losses, this number is dominated at lowfrequencies by the capacitor's dielectric loss. The capacitor'sdielectric loss is generally related to its dielectric constant and thefrequency of the driving energy. For example, if the frequency of anapplied sinusoid is relatively low (say 60 Hz) then the crystal latticeof the capacitor has plenty of time to deflect back and forth under theelectrical stress and in so doing, produces a significant amount of heatwhich is a type of real or resistive loss. At 1 kHz, the capacitordielectric structure (or dipoles, if one uses that theory) vibrates at ahigher frequency. As one goes higher and higher in frequency, say to 10MHz, then for the Class I dielectrics of the present invention, therewould be very little movement in the crystal lattice and accordingly,very little heat generated due to dielectric loss. It will be furtherillustrated how dielectric loss varies with frequency. In the past,particularly as described by testing specifications such as MIL-Std-202,dissipation factor is measured either at 1 kHz, or in some cases, at 1MHz. Unfortunately, this data is misleading at MRI RF-pulsed frequencieswhich generally are 21.28 MHz (0.5 T), 64 MHz (1.5 T) or higher. Formost dielectrics, the high frequency ohmic loss, due to the capacitor'selectrode plates, is so low that it is masked by the capacitor'sdielectric loss when measured at low frequencies such as 1 kHz or 1 MHz.This will be explained in subsequent figures.

FIG. 25 is a more complete schematic for a capacitor, which has beensimplified from FIG. 21. (R_(o)) represents ohmic loss 200 which is thesum of the connection loss (R_(e)) 184, the attachment materials (R_(a))186, the metallization (R_(m)) 188, and the electrode plate resistance(R_(e)) 190. Assuming that the connection resistance (R_(e)) 184 is verylow, such as in attachment to gold, and that the attachment material(R_(a)) 186 has a very low resistivity, such as a thermal-settingconductive adhesive or a solder, and assuming that the capacitormetallization 188 materials have very little ohmic resistance to theelectrode plates, then one can assume that the bulk of the entire ohmicloss (R_(o)) 200 is equal to the resistance of the electrode stack(R_(e)(total) 190. As previously described, the resistance of theelectrode stack depends on the length, the width and the thickness ofthe electrodes and importantly, also the number of electrodes that arein parallel. Therefore, reducing the dielectric loss and maximizing thenumber of electrodes are key featured embodiments of the presentinvention.

FIG. 26 is a simplified schematic diagram of the present invention fromFIG. 25 showing that the ESR 198 is the sum of the dielectric loss(R_(DL)) 192 plus the total parallel resistance of the electrode stack(R_(e)) 190. Referring once again to FIG. 25, one can see that there isa resistor (R_(IR)) 202 in parallel with the ideal capacitor C 194. Thisresistance (R_(IR)) 202 is known as the capacitor's insulationresistance. In a high quality capacitor, this resistance value tends tobe in the hundreds of megohms or higher and can therefore be ignored aspart of the equivalent circuit model for the purposes herein. Forthree-terminal or physically small MLCCs, the equivalent seriesinductance (ESL) 204 as shown in FIG. 25 is a very small value and canalso be ignored for the purposes herein. In addition, ESL 204 isimaginary and does not contribute to power loss or ESR 198 in acapacitor.

Accordingly, as shown in FIG. 26, the AIMD diverter capacitor 140′ ESR198 is the sum of the dielectric loss (R_(DL)) 192, the ohmic losses(R_(o)) 200 and any losses due to skin effect (R_(s)) 206. However, atMRI RF frequencies, skin effect is negligible and may be ignored.Referring once again to FIG. 26, assuming that the capacitor has goodmetallization, oxide free connection to the ferrule and good electricalattachment materials, then the ohmic losses (R_(o)) 200 are completelydominated by the resistance of the electrodes (R_(e)) 190. Accordingly,for the purposes of the present invention, the ESR 198 is generallyequal to the dielectric loss 192 plus the electrode losses (R_(e)) 190.Both of these parameters must be carefully controlled for the high powerRF diverter capacitor 140′ of the present invention.

It has been shown that dielectric loss is a frequency variable. At MRIRF pulsed frequencies, for an EIA Class I ceramic capacitor, thedielectric loss drops to a very low value (it is essentially zero).Therefore, in the present invention, which is based on EIA Class Idielectrics, the diverter capacitor's 140′ ESR 198 is primarilydetermined by the total resistance of its electrode plates (R_(e)) 190.

FIG. 27 illustrates the dielectric loss in ohms for a relatively lowdielectric constant ceramic capacitor. One can see, at low frequencies,the dielectric loss in ohms can be over 100 ohms or even much greater.However, as one increases in frequency, one can see that the dielectricloss drops and is nearly zero at 64 MHz (1.5 T MRI scanner RF-pulsedfrequency).

FIG. 28 shows a U-shaped composite curve. It is the summation ofcapacitor ohmic loss which includes the total resistance of capacitorelectrode plates, electrical attachment materials, capacitormetallization, and electrical connection material. As one can see,ignoring skin effect, the conductor ohmic loss for the capacitor isrelatively constant from low frequency all the way to very highfrequencies. For a Class I dielectric, the capacitor dielectric loss(marked with small squares) is a very high value at low frequency, andthen drops to near zero at MRI RF frequencies such as 64 MHz and 128MHz. Skin effect is also shown, which would be an ohmic loss fortwo-terminal type capacitors. The total ESR is the solid line, which isthe summation of the capacitor dielectric loss, the capacitor conductorohmic loss and skin effect. The present invention is directed to makesure the center of this U-shaped curve falls on the range of MRIRF-pulsed frequencies.

FIG. 29 is a table showing an example of losses (actually measured) fora prior art 2000 picofarad X7R (2500 k) feedthrough capacitor. Thisparticular capacitor had a dielectric constant of about 2500. One cansee that at 1 kHz, the dissipation factor is about 1591.55 ohms, whichwhen added to the ohmic losses, results in an equivalent seriesresistance of about 1591.98 ohms. Even at 1 MHz for this capacitor,there is about 1.59 ohms of dissipation factor loss, which when added tothe about 0.432 ohms of ohmic loss, yields an ESR of about 2.024 ohms.As one can see, again referring to MIL-Standard-220 and many other testspecifications, measuring the capacitor's real losses, at 1 kHz and 1MHz, is not a useful way to analyze the capacitor's losses at MRIRF-pulsed frequencies. For this, one needs to look in the range from 10to 500 MHz and realize that as the dissipation factor drops, the ohmiclosses still dominate and one ends up with a significant ESR rangingfrom about 0.59 to about 0.434 ohms.

FIG. 30 dramatically illustrates the difference when one uses an EIAClass I dielectric, such as COG (NPO), which has a dielectric constantof less than about 200. Because of this low dielectric constant, one isforced to use a very high number of electrode plates. This has theeffect of greatly reducing the capacitor's ohmic losses. In addition,Class I dielectrics have a lower dissipation factor, particularly athigh frequency. Comparing 100 MHz, one can see for the COG dielectric,the ESR is about 0.201 ohms at 100 MHz, which is a significant reductioncompared to the X7R capacitor. In the preferred embodiment (illustratedin FIGS. 32-70), the ESR would drop to below 0.1 ohms, which wouldresult in a significantly reduced heat generation in the presentinvention diverter capacitor 210.

FIG. 31 is a scan of the capacitor's ESR taken from an Agilent MaterialsAnalyzer. At the start frequency of 1 MHz, one can see that thecapacitor's 210 ESR is on the order of 6 ohms, which is very high.However, by using a EIA Class I dielectric, by the time one reachesabout 21.28 MHz (the frequency of a 0.5 T MRI scanner), the dielectricloss has flattened out. The only loss left is the ohmic loss of thecapacitor, which at 100 MHz is only 200 milliohms. Also shown are theRF-pulsed frequencies for a 1.5 Tesla scanner (64 MHz) and a 3 Teslascanner (128 MHz).

Since the 1960s it has been a common practice in the capacitor industryto measure capacitance and dissipation factor at 1 kHz. The dissipationfactor is usually defined as a percentage, for example, 2.5% maximum.What this means is that the dielectric loss resistance can be no morethan 2.5% of the capacitive reactance at a certain frequency (usually 1kHz). For example, if the capacitive reactance for a particularcapacitor was 80,000 ohms at 1 kHz with a 2% dissipation factor thiswould equate to 1600 ohms of resistance at 1 kHz. FIG. 31 alsoillustrates that the dielectric loss essentially goes to about zero athigh frequency. For typical low dielectric constant Class 1 ceramiccapacitors, frequencies above 10-20 MHz will be sufficiently high sothat the dielectric loss is no longer a factor in the capacitor ESRmeasurement. In summary, the ESR of the capacitor 210 varies with thecapacitance value, the number of electrode plates, and the length andwidth of the electrode plates. Accordingly, a wide range of “normal” ESRreadings can be obtained for many types of capacitors. For oneparticular capacitor a normal ESR reading might be 0.05 ohms and foranother design as much as 10 ohms.

In the present invention, as shown in the embodiment of FIG. 32,maximization of the number of electrode plates in order to reduce theelectrode resistance (R_(e)) becomes paramount. In general, in order toincrease the number of electrode plates, the effective capacitance area(ECA) can be minimized and the dielectric constant lowered so that oneends up with a relatively high number of electrode plates. One mightask, why doesn't one simply make the electrode plates much thicker inorder to decrease their resistance? It would be true that making theelectrode plates very thick would reduce their resistance, however,there would be an undesirable consequence. The capacitor would no longerbe a monolithic layer and would simply represent a sandwich somewhatlike a deck of cards that is ready to come apart at the first thermalshock or piezoelectric effect. It is a basic tenet of ceramicengineering that electrodes be thin enough, and contain enough ceramicpowder such that when sintered, the ceramic capacitor structure becometruly monolithic. This leaves the designer with only a few effectiveways to control the capacitor's ESR. For a given geometry, which isusually dictated by the AIMD design, there are very few degrees offreedom in the length, width and geometry of capacitor electrode plates.Accordingly, in the present invention, maximizing the number ofelectrode plates becomes a key design factor. This goes hand in handwith the capacitor's dielectric constant k. In other words, reducing thedielectric constant means that the number of capacitor electrode platesmust increase to achieve the same capacitance value. This naturallyreduces the capacitor's ESR and increases its ability to handle highlevels of RF current. Another reason to keep the ESR 198 of the divertercapacitor's 210 extremely low is so it does not overheat while divertinghigh levels of RF current to the EDS housing 124 of the AIMD 100. The RFcurrents are literally conducted through the capacitor's 210 electrodeplates 212, 214 and hence through the electrode plate resistance (R_(e))190. Electrode plate resistance (R_(e)) 190 is the sum total of theresistance of all of the electrode plates 212, 214 acting in parallel.If the electrode plate resistance (R_(e)) 190 were high, then therewould be a tremendous amount of I²R power loss that occurs and thecapacitor 210 would rapidly get very hot and perhaps destroy itselfand/or the surrounding electrical connections or materials. Anotherreason to keep the capacitor 210 ESR 198 relatively low is so that itrepresents a very low impedance Z at the MRI RF pulsed frequency. Thiswill increase its ability to draw energy from the implanted lead 110 anddivert it as an energy dissipating surface to the AIMD housing 124. Ifthe capacitor represented too high of an impedance, this would reducethe current, but would also mean that more energy was undesirably leftin the implanted lead 110. Lowering the impedance Z of the divertercapacitor 210 also means that it will be a better EMI filter by offeringincreased attenuation at the MRI RF pulsed frequency.

However, the most important reason of all to keep the overall resistance190 of the electrode plates extremely low (in other words, extremely lowESR) is to prevent the overheating of the primary filter capacitor 210itself. It has been demonstrated that overheating of this capacitorcauses the adjacent AIMD housing 124 to overheat. This is highlyundesirable in a human incision pocket. Typically, the AIMD is placedunder the skin, under the fat or even under a muscle. There are variousFDA and CEM42 Standards that limit the amount of heating in varioustypes of body tissues. In general, the amount of this heating is limitedto 4° C. (that can vary with body tissue). For example, for a deep brainstimulator, a subdural implanted AIMD must have a much lower temperaturerise due to the extreme thermal sensitivity of brain matter. This is incontrast to a pectoral pocket created for cardiac pacemaker or ICD,which represents less thermally sensitive tissues and fats. In anyevent, it is a major feature of the embodiments herein to prevent theoverheating of the primary filter component in order to minimize oreliminate AIMD can 124 heating.

In general, the filter capacitor 410, 210 of the present invention mayhave at least 10 electrode plates. However, with a k up to one thousand,one could envision a design of a capacitor having an intermediatedielectric constant of say 400. In this case, it might be possible todesign a capacitor with 5 electrode plates (depending upon their lengthand width) and still have a low enough ESR in accordance with thepresent invention. (For example, 5 active electrode plates with 5 groundelectrode plates.) Alternatively, the number of electrode plates couldbe as high as 20, 40 or even 100 or more, but the critical parameter isthat the capacitor's equivalent series resistance never exceeds 2 ohmsat the MRI RF-pulsed frequency. In various embodiments, the ESR would be<0.5 ohms or 0.1 ohms.

FIG. 32 illustrates a cross-section of a multilayer ceramic capacitorMLCC 210 of the present invention which is very similar to the prior artMLCC 140′ illustrated in FIGS. 14, 19 and 20. FIG. 32 can also beequivalent to any of the aforementioned feedthrough capacitors. In thepresent invention, feedthrough capacitors or MLCCs can act as high powerRF energy diverters. Energy diverters using an energy dissipationsurface 134, 124 are more thoroughly described in Published ApplicationNos. 2010/0217262 and 2010/0023000, the contents of which areincorporated herein by reference. The key difference is that the numberof electrode plates, both active 212 and ground 214, has beensubstantially increased in order to reduce the capacitor's 210 ESR 198at the MRI RF pulsed frequency to below 2 ohms. In a particularlypreferred embodiment, the capacitor's ESR 198 would be below 1 ohm. Aspreviously mentioned, a way to accomplish this without the capacitancevalue becoming too high would be to decrease the dielectric constantsuch that a high number of electrode plates would be required. In aparticularly preferred embodiment, the dielectric material would be anEIA Standard Class I type such as NPO. Referring once again to FIG. 32,one can see the active (left hand) electrode plates 212 and the groundelectrode plates (right hand) 214 stacked in interleaved relation. Anelectrical attachment material 152 is shown which connects the capacitormetallization 164 a and 164 b to the ferrule of a hermetic terminal 134.In general, the electrical connection material 152 would be highlyelectrical conductive, but not necessarily highly thermally-conductive.In summary, the capacitor 210 embodied in FIG. 32 is based on an EIAClass I dielectric, which means its dielectric constant is relativelylow and its temperature coefficient, as given by standardANSI/EIA-198-1, published Oct. 29, 2002, with reference to Table 2permissible capacitance change from 25 degrees C. (ppm/degree C.) forClass I ceramic dielectrics. This indicates that the maximum allowablechange varies from +400 to −7112 parts per million per degreescentigrade. As previously mentioned, a particularly preferred embodimentwould be the COG dielectric, which is also commonly referred to as NPO.

FIG. 33 is an equation showing the effect of the parallel plateresistances. FIG. 33 gives the equation for the total resistance of thecapacitor's electrode plates (R_(et)) 190 as the parallel summation ofall of the capacitors' electrode plates 212, 214 (“n” electrode plates).

FIG. 34 is very similar to the cross-section of the quad polar capacitorpreviously described in FIG. 9-10. Again, the number of electrode plates212, 214 have been increased in accordance with the present inventionsuch that the FIG. 34 quad polar diverter capacitor 210′ has a highfrequency ESR 198 generally less than 2 ohms. Referring once again toFIG. 34, one can see that the capacitor outside diameter (ground)metallization 164 is attached using a conductive material 152 to a goldsurface 158 on ferrule 134. All of these connections, when properlydone, have negligible resistance. Accordingly, the capacitor's 210′ ESR198 at high frequency is made up of the total of the resistance (R_(e))190 of the ground electrode plates 214 and the resistance (R_(e′)) 190′of the active electrode plates 212 all acting in parallel. As previouslystated, for Class I dielectrics, the capacitor's dielectric loss 192 canbe ignored at MRI RF pulsed frequencies since it becomes negligible atRF-pulsed frequencies. Also, for a feedthrough capacitor geometry, skineffect 206 is also negligible. Referring once again to FIG. 7-8, one cansee a similar rectangular quad polar capacitor that is attached to agold braze surface 158.

FIG. 35 is taken from section 35-35 from FIG. 34 and illustrates adoubling of the capacitor's active 212 and ground 214 electrode plates.Doubling the electrode plates 212, 214 is very effective since bothplates are still exposed to the capacitor's internal electric fields andtherefore, both sets of doubled plates will have electrode platedisplacement currents (RF currents). This has the effect of greatlyincreasing the number of electrode plates as illustrated in the equationin FIG. 33, which significantly reduces the overall electrode plateresistance. Dual electrodes are shown in U.S. Pat. No. 5,978,204 toStevenson el al., the contents of which are incorporated herein byreference. In the '204 patent, the dual electrodes were utilized tofacilitate high pulse currents, for example, in an implantabledefibrillator application. Double electrodes are very useful in thepresent invention to drive down electrode plate resistance, therebydriving down the capacitor's 210′ high frequency ESR 198 and also toincrease the conduction of RF energy and/or heat 218 out of thecapacitor 210′ during exposure to high power MRI RF-pulsed environments.

FIG. 36 is very similar to FIG. 35 except in this case, only the groundelectrode plates 214 have been doubled. Increasing the number of groundplates 214 is particularly efficient in the removal of heat. As shown,the ground plates 214 are utilized to conduct heat away from thediverter capacitor 210′ and direct it through the ferrule of thehermetic seal 134 to the housing 124 of the AIMD 100, which has arelatively large surface area. The relatively large surface area of theAIMD 100 means that a great deal of RF or thermal energy can bedissipated without concentrating it in a small location, which wouldlead to a very high temperature rise and possibly damage surroundingbody tissue damage.

FIG. 37 illustrates a family of lowpass filters 260 that all incorporatediverter capacitors 210 of the present invention. As can be seen, theselowpass filters 260 incorporate a variety of capacitors 210 ranging froma simple MLCC chip capacitor “C” to a 3-terminal “feedthroughcapacitor-FTC”. These capacitors 210 can be combined in various wayswith inductors to form “L,” “reverse L,” “T,” “Pi,” “LL,” or “reverseLL” or “n-element” lowpass filters. In other words, any of the highpower RF handling diverter capacitors of the present invention can becombined with any of the lowpass filter circuits as illustrated in FIG.37 for the purpose of protecting AIMD electronics from EMI while at thesame time pulling MRI induced energy from an implanted lead.

FIG. 38 is similar to FIG. 82 from publication 2014/0168850 (whichillustrated an electrical schematic embodying an AIMD where the leadenters the AIMD at a hermetic seal 132 and then encounters the low passfilter elements of 260 of FIG. 37) except in this case, the generallowpass filter 260 is in its simplest form. In this case, the generallowpass filter 260 is a feedthrough capacitor 210′ which is in turn,connected in series with a bandstop filter 258 which is in turnconnected with an L-C trap filter 262 disposed between the circuit traceor lead wire and the AIMD housing 124.

FIG. 39 shows a dual chamber bipolar cardiac pacemaker 100C with leadsimplanted into the right atrium and right ventricle of the heart 112. Asshown, header block 138 comprises industry standard IS-1 connectors 126,128. MRI energy is shown being induced on the implanted leads 110 and110′. As this energy enters the pacemaker housing 124, it encountersdiverter capacitor 210′. The diverter capacitor 210′ is designed todissipate high RF power in accordance with the present invention.Accordingly, diverter capacitor 210′ has a low dielectric loss at highfrequency and also very low high frequency ESR. In this case, there is asecondary row of MLCC chip capacitors 210 a through 210 d that aremounted at a location distant from the primary diverter capacitor 210′.In this case, the primary diverter capacitor could have a lowercapacitance value and the rest of the capacitance is comprised of eitherboard mounted capacitors 210 a through 210 d or the like. As shown, thecircuit board comprises a ground circuit trace 182 that is connectedthrough a low impedance RF conductor or strap 264 conducted to the AIMDhousing 124. This low impedance is important to conduct MRI RF currentsefficiently to the housing 124 of the AIMD. In order to spread out heat,multiple straps 264 can be used (not shown). A major advantage of thestructure shown in FIG. 39 is that by spreading out the filteringfunction, RF heat or MRI RF energy induced heat is dissipated or spreadout over much larger areas. This avoids hot spots on the AIMD housing124.

FIG. 39A is the electrical schematic of one of the leadwire circuits 136a of the cardiac pacemaker of FIG. 39. The first low ESR feedthroughcapacitor 210′ is shown in parallel with the 2-terminal capacitors 210a. It will be appreciated that all four of the quad polar leads 136 athrough 136 d have the same schematic parallel construction.

Referring once again to FIG. 39, one can see that the at least twoleadwires 136 a through 136 d extend from the body fluid side throughthe hermetic terminal 154 in non-conductive relationship and thenthrough feedthrough capacitor 210′. These leadwires then extend downinside the AIMD housing 124 to either via holes or circuit trace landson circuit board 130. The other end of the MLCC chip capacitor 210 athrough 210 d is electrically connected through the landing pad or tothe via hole to the leadwires 142 a-d. For the purposes of thisinvention, we will refer to the internal electrode plates of the MLCCchips 210 as having ground and active electrode plates. The groundelectrode plates are connected to the capacitor's end termination andare therefore, electrically connected to the circuit trace 182. Thecapacitor's active electrode plate set is electrically connected to theat least two leadwires 142 a-d. It will be appreciated by those skilledin the art that circuit board 130 could be an alumina ceramic board, itcould be a single layer board, it could be a multilayer board, it couldbe made of fiberglass or FR4 or any number of materials that circuitboards are made of. It will also be appreciated that a connection fromthe active side of the capacitors 210 could be accomplished by a flexcable (not shown). The flex cable would replace the leadwires 142 athrough 142 b on the inside of the AIMD housing (or inboard side) andthe flex cable would connect to shortened leadwires adjacent thefeedthrough capacitor 210′. The use of a flex cable greatly simplifiesand facilitates the assembly of the AIMD internal circuits. It shouldalso be noted that the circuit traces 182 and the embedded circuittraces 178 of circuit board 130 (and other circuit traces not shown) canbe made from a variety of materials. Since these are inside thehermetically sealed and biocompatible AIMD housing 124, the circuittraces need not be biocompatible themselves. In fact, they could be madeof copper, silver, platinum or any other highly conductive material. Inan alternative embodiment (FIG. 39B), the chip capacitors 210 a couldalso be mounted directly to the flex cable instead of to the circuitboard 130.

FIG. 39B illustrates a unipolar hermetic terminal assembly consisting ofa ferrule 134, an alumina insulator 156 and a leadwire 136 on the bodyfluid side wherein, the same leadwire is labeled 142 on the inboard side(or the body side). Shown is a flex cable 171 wherein, one of thecircuit traces is connected to the hermetic pin of the feedthrough.There is also a second circuit trace which is connected to ground, whichis the potential of the ferrule 134. Importantly, the ferrule 134 iswelded to the AIMD housing 124. The chip capacitor 410, which has a k of<1000 in accordance with the present invention, is shown electricallyconnected between the grounded circuit trace 182 and the active circuittrace 143, which is electrically connected to the feedthrough pin 142,136. Electrical connection material 152 is shown, which can be a solder,a thermal-setting conductive adhesive or the like.

Referring once again to FIG. 39, one can see that the chip capacitors210 are considerable distance from the point of leadwire ingress throughthe hermetic terminal ferrule 134. The structure shown in FIG. 39B putsthe MLCC capacitor 410 closer to the point of leadwire ingress 142. Thisis also the point of ingress of undesirable EMI signals that may bepicked up on an implanted lead. Having the chip capacitors as close aspossible to the hermetic seal is desirable since it cuts down theinductance or inductive loop inside the device. This helps to preventthe so-called “genie-in-the-bottle” effect wherein, once EMI is insidethe AIMD housing, it can cross-couple, reradiate or couple throughantenna action to sensitive electronic circuits thereby causingdisruption. At MRI RF-pulsed frequencies, this is not a particularconcern since for a 1.5 T scanner, the RF-pulsed frequency is 64 MHz.The wavelength of a 64 MHz signal is so long that it really does noteffectively re-radiate once inside an AIMD housing. However, if the MRIfilter capacitor 410 is also to be used as a broadband low pass filter,for example, where it must filter out very high frequency signals above1 GHz, such as those signals from cellular telephones, then it isdesirable to have the chip capacitor 410 as close as possible to thepoint of leadwire ingress. Using a chip capacitor for both diverting ofMRI RF-pulsed frequencies and also to act as a broadband low pass filtermeans that one desirably places the MLCC capacitor as close as possibleto the point of leadwire ingress. This is shown in FIG. 19 withcapacitors 140, which of course, could be capacitors 410 in accordancewith the present invention. One is also referred to FIG. 54, whichplaces the MLCC capacitors 410 directly at the point of leadwire ingressof the AIMD housing where the capacitor 410 is connected to the terminalpin and to the gold braze of the ferrule, thereby providing the lowestimpedance connection possible. Mounting chip capacitors directly at thepoint of leadwire ingress is further taught by U.S. Pat. Nos. 5,650,759and 5,896,267, the contents of which are herein incorporated byreference.

FIG. 40 shows an alternative embodiment to FIG. 39. A circuit board andchip capacitors 210 a through 210 d as previously described in FIG. 39are shown. However in this embodiment, the grounded circuit trace 182does not need a ground strap or conductor 264 to the AIMD housing.Instead, a shielded conduit assembly 266 is attached to the ferrule ofthe hermetic terminal (not shown). This shielded conduit 266 is groundedwith a strap 268 which is connected to the ground circuit trace 182.This type of EMI shielded conduit assembly is more thoroughly describedin U.S. Pat. No. 8,095,224 to Truex et al., the contents of which areincorporated herein by reference.

FIG. 41 shows a cross-sectional view of a flex cable or circuit board270. The flex cable or circuit board 270 is attached on the left using aball grid array (BGA) type attachment 254. Attachment 254 is furtherconnected to a conductor 142 that goes through a hermetic seal 132 of anAIMD (not shown). These types of flexible circuit traces or substratesare also described in U.S. Pat. No. 8,095,224 to Truex et al., thecontents of which are incorporated herein by reference.

FIG. 42 shows a cross sectional view generally taken from section 42-42of FIG. 41 and shows the conductive circuit traces 178 a through 178 d.

FIG. 43 illustrates a cross sectional view generally taken from section43-43 of FIG. 41 and shows an optional embodiment wherein a groundshield 182 surrounds the four circuit traces 178 a through 178 d.

FIG. 44 is a cross sectional view taken generally from section 44-44 ofFIG. 41 and illustrates shield layers 272 a, 272 b. These shield layers272 a, 272 b are designed to surround each of the circuit trace layers178 as previously described in FIG. 42 or 43. These shields 272 a, 272 bare not absolutely required, but greatly assist in preventingre-radiation of electromagnetic interference inside of the AIMD housing124. This re-radiation of EMI can be very dangerous as it can couple tosensitive AIMD circuits and disrupt the proper functioning of the AIMD.

FIG. 45 illustrates an embodiment in which the circuit traces 178 athrough 178 d of FIGS. 41 through 44 are connected to a circuit board orsubstrate 270. Electrical attachments 274 are made to active circuittraces and in turn to a multi-element flat-through diverter capacitor210. This three-terminal flat-through capacitor is very similar to thatpreviously described in FIGS. 24 and 25 except that it has fourcapacitors embedded in a single structure. Capacitor 210 may replace theindividual capacitor 210 a through 210 d as previously illustrated inFIG. 39 or capacitors 210 a through 210 d as previously described inFIG. 40.

FIG. 46 shows a top view of the flat-through diverter capacitor 210 ofFIG. 45.

FIG. 47 is a cross sectional view taken generally from section 47-47 ofFIG. 45 and shows the active electrode plates 212 of the flat-throughdiverter capacitor 219 of FIG. 45.

FIG. 48 is a cross sectional view taken generally from section 48-48 ofFIG. 45 and shows the ground electrode plate 214 set of the flat-throughcapacitor 210 of FIG. 45.

Accordingly, from all of the foregoing it will be appreciated that thisinvention addresses the problems created when the radio frequency (RF)pulsed field of MRI couples to an implanted lead in such a way thatelectromagnetic forces (EMFs), voltages and current are induced in thelead. The amount of energy that is induced is related to a number ofcomplex factors, but in general, is dependent upon the local electricfield that is tangent to the lead and the integral electric fieldstrength along the lead. In certain situations, these EMFs can causecurrents to flow into distal electrodes or in the electrode interfacewith body tissue. It has been documented that when this current becomesexcessive, that overheating of the lead or its associated electrodes canoccur. In addition, overheating of the associated interface with bodytissue can also occur.

There have been cases of overheated electrode damage to cardiac tissuewhich has resulted in loss of capture of cardiac pacemaking pulses.Furthermore, with respect to neurostimulators, neurological tissuedamage severe enough to result in brain damage or multiple limbamputations have also been documented.

The present invention relates generally to methods and apparatus forredirecting RF energy to locations other than the distal tipelectrode-to-tissue interface. In addition, the present inventionprovides electromagnetic interference (EMI) protection to sensitiveactive implantable medical device (AIMD) electronics. The redirection ofthis RF energy is generally achieved by the use of frequency selectivedevices, such as inductors, capacitors and filtered networks. Asdescribed in U.S. Pat. No. 7,689,288, to Stevenson et al., the contentsof which are incorporated herein by reference, filtered energydissipation networks can range from a single capacitor, such as afeedthrough capacitor, to more complex filters that may include L-Ctraps and/or L-C bandstop filters co-operating in various ways with C,L, Pi, T or n-element lowpass filters. In general, this is accomplishedthrough frequency selective lowpass filters or series resonant LC trapfilters wherein the RF energy can be redirected to another surface or isconverted to heat. In all of the above described frequency selectivenetworks, it is the capacitor(s) (co-operating with other circuitelements) which divert energy from an implantable lead conductor to theconductive housing 124 of an AIMD. The relatively large surface area ofthe AIMD housing 124 acts as an energy dissipating surface (EDS) whereina significant amount of the MRI energy can be harmlessly dissipatedwithout significant temperature rise. However, the lowpass filter alsoknown as diverter capacitor elements must be designed to handle a veryhigh amount of RF current and power. Accordingly, the capacitor'sinternal resistive or real losses known as equivalent series resistance(ESR) must be kept quite low. The present invention is directed tovarious embodiments of MRI diverter capacitor designs that minimize thediverter capacitor's equivalent series resistance (ESR). In addition,the capacitor is also designed to direct heat to relatively largesurface area heat dissipation surfaces, thereby creating an efficientheat removal system. These high RF power/low ESR diverter capacitors arean important feature of the filter network of the present invention fordiverting induced RF energy from an implanted lead conductor to anenergy dissipating surface, particularly a conductive housing 124 of anAIMD.

These implantable lead systems are generally associated with AIMDs, suchas cardiac pacemakers, cardioverter defibrillators, neurostimulators andthe like. The present invention can also be incorporated with externaldevices, such as external pacemakers, externally worn neurostimulators(such as pain control spinal cord stimulators), catheters, probes andthe like. It will be shown that for a given geometry constraint, apreferred means of reducing the diverter capacitor's ESR is to selectthe most ideal dielectric type so that its dielectric loss tangent(dielectric losses) is insignificant at the MRI RF pulsedfrequency(ies). Of particular importance in the present invention isselection of a capacitor dielectric with the proper dielectric constant(k) value. The preferred capacitor dielectric will have a k of asufficiently low value to thereby increase the number of active andground electrode plates in the capacitor. This design featuredramatically reduces the ohmic losses in the capacitor at highfrequency. Therefore, to accomplish a relatively high electrode platecount, a low k capacitor dielectric is used. A non-limiting example ofone such dielectric material is an EIA standard, Class I dielectricmaterial, COG, which is also known as NPO (negative-positive-zero).(Refer to EIA Standard ANSI/EIA-198-1-F-2002).

In general, at first glance, using a Class I dielectric iscounterintuitive. For example, consider a typical X7R MLCC dielectric,with a dielectric constant of around 2500. With such a high efficiencydielectric material having a relatively high dielectric constant, itwould be possible to build, for example, a 1000 picofarad filtercapacitor with two to four electrode plates. Now consider using a Class1 COG dielectric, wherein the dielectric constant is less than 100. Atypical capacitor comprising the COG dielectric material would generallyrequire greater than 20 or even 40 electrode plates to achieve the samecapacitance value. Such a design would, however, provide a capacitorwith a relatively large thickness and would also require significantlymore precious metal in its manufacturing. A capacitor of this design isgenerally not desired.

Nonetheless, the benefit of incorporating a COG dielectric materialwithin the capacitor design is generally a reduction of the capacitor'sESR at MRI RF-pulsed frequencies. If designed properly, the RF energyheat that is produced when positioned within an MRI scanner can besignificantly reduced such that heat that results from RF energy doesnot pose harm to biological tissue.

One purpose of these low ESR diverter capacitors and related lowpassfilter circuits is to provide electromagnetic interference (EMI)filtering in order to protect sensitive AIMD electronic circuits frommalfunctioning in the presence of MRI RF noise. Another purpose of thesecircuits, as described in the present invention, is to draw MRI inducedenergy out of the lead and redirect said energy to the AIMD housing.This has the effect of reducing the energy that would reach the distaltip electrode or the interface with body tissue. By redirecting saidenergy to locations at a point distant from the distal electrodes,ideally the AIMD housing, this minimizes or eliminates hazardsassociated with overheating of said distal electrodes during diagnosticprocedures, such as MRI.

For maximum RF energy transfer out of the lead, frequency selectivediverter circuits are needed which decouple and transfer energy which isinduced onto implanted leads from the MRI pulsed RF field to an energydissipating surface. Importantly, while decoupling and transferring suchenergy, it is extremely important that the diverter circuits do notthemselves overheat thereby creating hot spots on the AIMD housing,which could damage tissue, for example, in a pacemaker pectoral pocket.Recent experiments by the inventors have seen temperature rises from 4to 10 degrees C. on the pacemaker housing directly over the location ofthe feedthrough capacitor during a 4 watt/kilogram MRI scan. In general,in the prior art, MLCC capacitors are really not indicated for highpower RF applications. The reason for this is that the impedance(capacitive reactance) drops so low that extremely high RF currents endup flowing through the capacitor's electrode plates. During a 4watt/kilogram MRI scan where 16 to 20 volts may be induced at the AIMDinput, the diverter capacitor may be handling anywhere from 0.5 to 4amps of RF current. If the ESR of the capacitor, for example, was 0.5ohms and the capacitor was diverting 2 amps, then the I²R loss would beon the order of 2 watts. Two watts of dissipation on this smallcomponent would cause it to overheat significantly. The presentinvention fulfills these needs and provides other related advantages.

The RF diverting circuits, in general, conduct MRI induced RF energyfrom the lead or its associated lead wires to an EDS such as the housing124 of the AIMD. The design of the diverter circuit is very important.First of all, the diverter circuit should appear as a very low impedanceat MRI RF frequencies such that a maximum amount of RF energy isdiverted from the lead conductor to the EDS. In addition, it is alsodesirable that the diverter element be designed such that it does notoverheat.

Furthermore the mounting location of the diverter capacitor within anAIMD is also typically constrained by proper EMI design practices.Generally, EMI filters are designed such that undesirable RF energy isdiverted at the point of lead ingress to the AIMD housing, as opposed toletting the EMI inside the AIMD housing and trying to filter it furtherdownstream, such as on an internal circuit board. In a preferredembodiment, at least one of the low ESR diverter capacitors of thepresent invention is mounted directly to the multi-pin hermetic sealterminal of the AIMD. This is an ideal location both to divert RF energybefore it can enter the AIMD housing but is also optimal for heatconduction and dissipation. Even with low ESR, the diverter capacitorwill still be dissipating a significant amount of energy. This means,even with low ESR, the diverter capacitor is creating heat which must beconducted or convected away so that a hot spot does not occur on theAIMD housing at or near the filter capacitor. Therefore, by divertingboth the RF energy and heat to the relatively large surface area of thehousing of the AIMD the MRI RF energy can be dissipated with only asmall temperature rise that does not adversely affect body tissue.

It should be pointed out that the general principle of placing theprimary filter capacitor (energy diverter) at the point of leadwireconductor ingress in the AIMD housing is generally the preferred EMIdesign practice. For relatively low frequencies, such as an MRI RFpulsed frequency of 64 MHz, it would be perfectly acceptable to placethe primary filter diverter capacitor on a circuit board remote from thehermetic terminal otherwise known as the point of leadwire ingress. Insummary, having primary low ESR diverter capacitors only on a remotecircuit board is not the optimal way to, at the same time, provide forhigh frequency filtering of AIMD electronics, but it would be acceptablefor MRI compatible AIMDs, such as MRI conditionally approved pacemakers.

FIG. 49 illustrates a family of low pass filters, which is very similarto the family of low pass filters described in FIG. 37. Referring onceagain to FIG. 49, these are also known as EMI filters. This is becauseEMI filters are low pass filters, meaning that they allow lowfrequencies to pass and provide a substantial amount of attenuation tohigher frequencies. In every one of the circuits of FIG. 49, there is acapacitor element 410 that is directed towards the implantable leadelectrode (body fluid side). This is the path from an electrode, whichis contacted with biological cells along the conductor of an implantablelead through the hermetic seal of the AIMD and along that sameconductor, directly to capacitor 410. In accordance with the presentinvention, capacitor 410 must be a very high RF power, a low ESRhandling capacitor so that the capacitor 410 and hence the AIMD will notoverheat in an MRI environment. One will also note that the “n” elementfilter has been revised so that there is no longer an inductor directedtoward the implanted lead electrode. The version of the LL filter, withthe inductor directed to the implanted lead electrode has also beeneliminated. In addition, the version of the 2-element or L filter, withthe inductor toward the implanted lead electrode has also beeneliminated. This is because, in the present invention, it is veryimportant that the primary high power RF handling capacitor have adirect connection from its active electrode plates through the hermeticseal to the one or more electrodes of an implantable lead.

Referring once again to FIG. 49, for the n-element, LL and Pi( )filters, one can see that there are two capacitors 410 and 140 separatedby an inductor. In the present invention, it is critical that capacitor410 have an ESR <0.5Ω at the MRI RF pusled frequency and a dielectricconstant <1000. This is in order that it have high electrode plate countand have a very low equivalent series resistance at MRI pulsedfrequencies. The capacitor element 140 could be constructed of a low ESRconstruction the same as capacitor 410 or it could be constructed asprior art filter capacitors have been constructed in the past and thatis with conventional ceramic dielectrics with a k>1000. Another way oflooking at this is that the first capacitor directed toward theimplanted electrode is the work horse and is going to do the bulk of thediverting of RF energy from the lead and diverting it to the AIMDhousing where the energy and or heat could be dissipated over a largesurface area.

The first EMI filter ever designed for an active implantable medicaldevice was in the mid-1970s for the Xytron Medtronic pacemaker. Thesewere unipolar feedthrough capacitor EMI filters that had a k of above1200. The principle designer on this filter design project was RobertStevenson, one of the co-inventors herein. The next EMI filter to bedesigned for cardiac pacemaker was in 1979 for a St. Jude pacemaker.Robert Stevenson worked with St. Jude Vice President Buehl Truex todesign in this filter, which generally had a k above 2200. The inventorsherein have spent their entire careers designing EMI filters for avariety of applications, including AIMD applications.

There has never been a case where the primary (work horse) passive EMIlow pass filter has had a k below 1200. In addition, the inventors haveeither been asked to bid, have been aware of, or have cross-sectionedand analyzed explants of other manufacturer's EMI filters and have foundthe same thing to be true and that is, they have been built around adielectric structure that has a k of at least 1200. For the last 30years, almost all the primary EMI filters (the work horse filter) hasbeen designed with a k of greater than 1200.

There are several reasons why the industry has always been at a k ofabout 1200 and generally above 2100 to 2200. The first important reasonis that active implantable medical devices must be very small in sizeand very low in weight. Another consideration is cost. By using a high kdielectric, one needs fewer electrode plates and can build the capacitormuch thinner and in a much smaller overall volume or footprint. This isideal for all AIMDs, again where size and weight are critical. Until theMRI application came along, which this patent identifies, it was nevercontemplated to do what is completely counter-intuitive and that is touse a lower k capacitor.

The parent invention claims primary (work horse) passive divertercapacitors of less than 200 k. There is a general reason for this andthat is the major material suppliers in the ceramic dielectricindustries, such as Ferro offer dielectrics above 1200 k or below 200 k.The dielectrics below 200 k are known as Class 1 dielectrics. TheseClass 1 dielectrics find broad application in military and spaceapplications that have never been used in the past for the primary EMIlow pass filter capacitor for an AIMD. There is a vast desert in termsof material supply in the industry and that there are almost nosuppliers of dielectric materials between 200 and 1000 k. There are acouple of specialty ceramic powder manufacturers, one of which is calledDimat, Inc. They offer a range of specialty dielectrics, including anN2200, which has a dielectric constant of 250; an N3300, which has adielectric constant of 400; an N4700, which has a dielectric constant of600; and, an N5250, which has a dielectric constant of 700. There isalso another company called MRA Materials, which offers a dielectricswith a k of 485 and also a dielectric with a dielectric constant of 600.

First of all, none of the specialty or niche dielectrics have ever beenused as the primary low pass EMI filter for an AIMD. The presentinvention claims that the primary EMI filter capacitor, which isdirectly connected through wiring to an implanted lead conductor withdistal electrodes, be of less than 1000 k. There is a practical reasonfor this. In some cases, the capacitance value can be considerably high,such as 1800 pF. Building this capacitor out of a common commerciallyavailable dielectric, such as NPO (with a k of 90) results in acapacitor that has so many electrode plates that it becomes too thick tofit into the cardiac pacemaker. Accordingly, the inventor's developed anintermediate k dielectric (between 200 and 1000 k), which will presentan ideal tradeoff between volumetric efficiency, a lower k, a highernumber of electrode plates and accordingly, an ESR of less than 0.5 ohmsthat will meet all of the design criteria, including small in size, lowin weight and low in cost.

In the embodiments herein, it is also possible to split the function ofthe primary or work horse diverter or low pass filter capacitor andbreak it up into two areas. One is referred to FIG. 39, which shows thatthe primary diverter capacitor can be broken up into two differentcapacitors, such as a feedthrough capacitor 210′ and a board mountedcapacitor 210. In a particularly preferred design, the capacitor 210would have a dielectric constant of less than 1000 and even preferablyless than 200. And for a needed capacitance value overall of 1800 pF,the high-energy, low ESR capacitor 210′ could be 800 pF and the boardmounted capacitors, which could be conventional technology with adielectric constant above 1000, could be 1000 pF. These capacitors wouldadd up in parallel to give 1800 pF, which is the design goal, but inthis case, capacitor 210′ is thinner thereby facilitating packagingbetween said capacitor and the circuit board structure. The values of800 and 1000 pF are chosen at random and are not necessarilyrepresentative of any particular design. In other words, capacitor 210could be 200 pF and capacitors 210 a could be 1600 pF or any otherpossible combination one could imagine.

In summary, there has never been a primary filter (work horse) capacitorwith a direct connection to the conductor of an implantable lead with anelectrode in contact with body tissues ever built with a dielectricconstant of less than 1000. It has been pointed out that one of thereasons for this is that designing a capacitor with a dielectricconstant of less than 1000 is completely counter-intuitive forincorporation into AIMDs. It is only the advent of MRI compatiblepacemaking systems and leads and the recent discovery that the primaryfilter capacitor itself can overheat and lead to excessive AIMD can(housing 124) heating when implanted in the human pocket that makesprimary filter capacitors with a k less than 1000 suddenly an attractivedesign solution. This is because the dielectric constant of thesecapacitors when below 1000 leads to such a high number of electrodeplates that the capacitor's ESR is so low that when the capacitordiverts (up to 6 amps) MRI RF frequencies, it will itself not overheat.The capacitor's ESR at high frequency is primarily an ohmic loss andgiven that all other electrical connections are solid, the ESR islargely correlative to the resistance of the electrode plate. When oneincreases the number of electrode plates, this ESR resistance dropssignificantly.

In the embodiments herein, the goal is to drop ESR so much thatinsignificant heat is produced by the filter capacitor itself so thatundue AIMD implant pocket heating does not occur. The FDA and theindustry generally limit implant pocket heating to about 4 degreesCentigrade. It has been demonstrated by the inventors that thecombination of overheating of prior art primary low pass filtercapacitors with a k greater than 1200 can by themselves result in theAIMD housing and the corresponding human pocket overheatingsignificantly above 4 degrees C. The herein disclosed embodiments solvesthat problem and has other related advantages.

Referring once again to FIG. 37, there is an inconsistency between the“LL” and “reverse LL” as compared to the “L” and “reverse L.” Referringto the LL filter, one can see that there is a capacitor 210 on the leftside of the circuit. For the reverse LL, there is an inductor on theleft side of the circuit. This is somewhat inconsistent with theteachings of the L filter wherein, the inductor is first directed to theleft side (as opposed to the capacitor 210 for the LL configuration).For the reverse L filter, capacitor 210 is directed to the left. Therereally is no industry standard on what constitutes a reverse L or areverse LL. In fact, when manufactured, they could be installed by theuser in either direction, which would further confuse the issue.Accordingly, in the present invention, it is shown both ways to indicatethat there really is no standardization between the words LL and reverseLL or L and reverse L. The reference in the claims to the LL, thereverse LL, the L and the reverse L filters refer to the specificelectrical schematic shown in FIGS. 37 and 49 and also in other detailedschematic drawings.

Referring once again to FIG. 49, one will see that for the singleelement capacitor, it can be a two-terminal device 410 “C” or a singleelement feedthrough capacitor “FTC.” It will be understood by thoseskilled in the art that any of the capacitors 410 shown in FIG. 49,could be two-terminal capacitors or feedthrough capacitors “FTC.”

Referring once again to FIG. 49, one can see that the insertion lossversus frequency curve for the three-terminal feedthrough capacitor ofFTC does not have a significant resonant dip versus frequency. However,when one refers to the insertion loss curve for a single element MLCCchip capacitor 410, one can see that it has a significant self-resonancef_(r) as shown. This resonant frequency f_(r) is because in thecapacitor equivalent circuit, as previously shown in FIG. 25, the MLCCchip capacitor has significant equivalent series inductance 204. When acapacitor has equivalent series inductance, there is always going to besome frequency at which the capacitive reactance is equal and oppositeto the inductive reactance. This is known as the capacitor'sself-resonant frequency. One can see that there is an insertion losspeak at the capacitor's self-resonant frequency f_(r). This would go toinfinity if it were not for the capacitor's equivalent seriesresistance. In other words, this would go to infinite attenuation in dBif not for the capacitor's equivalent series resistance (ESR). Theproblem for the MLCC capacitor occurs at frequencies above itsself-resonant frequency f_(r). At higher frequencies, above theself-resonant frequency, the inductive reactance becomes increasinglydominant, which undesirably reduces the filter attenuation in dB. Wellmounted feedthrough capacitors tend to have essentially zero equivalentresistance 204, as shown in FIG. 25 and therefore, have a more idealfilter response curve versus frequency.

FIG. 49A reads on FIG. 39 and FIG. 39A. In FIG. 39, it was describedthat one could split up the primary filter capacitance into afeedthrough capacitor 210′ and a board-mounted capacitor 210 a.Referring once again to FIG. 49A, the schematic diagram is redrawnshowing a feedthrough capacitor 410 in accordance with the presentinvention wherein, the feedthrough capacitor has a k<1000 and extremelylow ESR properties (an ESR of <2 ohms at the MRI RF-pulsed frequency).The circuit board-mounted MLCC chip capacitors (410, 210, 140) are shownin parallel with the primary feedthrough capacitor 410. The MLCC chipcapacitor 410, 210 or 140, can have a k of <1000 in accordance with thepresent invention (410), can have a k<200 (210) or be a prior art MLCCcapacitor having a k>1000 (140). Also shown in FIG. 49A is the compositeinsertion loss curve of the feedthrough capacitor in parallel with theMLCC chip capacitor. One can see what would happen if we only have theMLCC capacitor as shown as a dashed line. However, the feedthroughcapacitor having zero series inductance picks up the degradation andinsertion loss that would occur from the MLCC only and thereforeprovides an overall/combined broadband low pass filter performance whichis ideal for an EMI filter and an MRI filter.

Referring back again to FIG. 49, one can see that each one of the lowpass filter circuits has a phantom inductor L′ drawn with dashed lines.This is in order to recognize that all conductors have some amount ofseries inductance and that some amount of inductance will be in seriesbetween the primary work horse capacitor 410 and the distal leadconductor electrode. It is very desirable in the present invention thatthis inductance L′ be kept as low as possible. If this inductor becametoo large, then a large inductive reactance would occur at MRI RF-pulsedfrequencies, thereby reducing the amount of energy that could be pulledfrom the lead. In other words, it is desirable that the first thing thatis connected to the implantable lead electrode along the path of theimplantable lead electrode be the work horse capacitor 410 so that itcan draw maximal energy out of the lead. It is also critical that thiswork horse capacitor 410 be very low in ESR so that it does not overheatwhile drawing literally amps of MRI induced energy out of the leadsystem. Many implantable leads themselves are made of spiraled or ofcoiled construction. Some of these are insulated and some of these arenot insulated. The uninsulated lead conductors tend to short together,particularly when going through torturous paths, such as bends in avenous system. Therefore, their parasitic conductance will varysignificantly due to design and lead trajectory differences. In summary,the phantom inductors L′, shown in FIG. 49, simply acknowledge that someparasitic inductance can be associated with the direct connection of theprimary work horse capacitor 410 and a distal electrode. It isparticularly important that the parasitic inductance be minimized fromthe point of leadwire ingress into the AIMD housing. That is, from theconductor that passes through the hermetic seal insulator innon-conductive relation, it is very important that there be very littleinductance or insignificant parasitic inductance between the work horsecapacitor 410 and that point of leadwire penetration into the AIMDhousing.

FIG. 50 illustrates that the high energy dissipating low ESR capacitor410 can be used in combination with other circuits, such as bandstopfilter 258 and L-C trap filter 262 consisting of capacitor 140 andinductor L. Again, the capacitor of a bandstop filter 258 and thecapacitor 140 of the L-C trap can be conventional prior art filtercapacitors. However, capacitor 410 is the work horse and must be verylow in ESR in accordance with the present invention.

FIG. 51 and FIG. 52 shows an MLCC capacitor that is similar in itsexterior appearance to the prior art MLCC capacitor previously describedin FIGS. 14 and 15. In accordance with the present invention, the MLCCcapacitor of FIGS. 51 and 52 has a k<1000 and an ESR of <0.50 at the MRIRF-pulsed frequency. Also, in accordance with the present invention, ithas a relatively high number of active and ground electrode plates 144,146 compared to the prior art chip capacitor previously illustrated inFIGS. 14 and 15. FIG. 53 is the electrical schematic of FIGS. 51 and 52showing in that a preferred embodiment the ESR is less than 0.5 Ohms.

FIG. 54 is a bipolar hermetic seal having a metallic ferrule 134 and twoleads 142 a and 142 b passing through the conductive ferrule ininsulative relationship. There are two MLCC chip capacitors 410 a and410 b, as previously illustrated in FIGS. 51 and 52. These chipcapacitors are electrically connected between each one of the respectiveleads and to the ground of the ferrule. Shown is an attachment to a goldbond pad/braze 154 a on the ferrule in order to provide an oxide freeand very low resistance electrical connection to the titanium ferrule134. This is more thoroughly described in U.S. Pat. No. 6,765,779, thecontents of which are incorporated herein by reference. FIG. 55 is aschematic diagram of the bipolar filtered hermetic terminal of FIG. 54.

Referring now back to FIG. 19, one will appreciate that the MLCCcapacitors of FIGS. 51 and 52 of the present invention could also beadapted to the substrate 147. That is, the chip capacitors could bemounted on the circuit board or substrate, which is then mounted to thehermetic terminal adjacent to the ferrule and/or insulator. Such circuitboards and substrates could be mounted immediately adjacent to theterminal or adjacent the hermetic terminal or even at a remote location.

FIG. 56 is similar to FIGS. 6 and 39 showing a breakaway cross-sectionof a typical AIMD, such as a cardiac pacemaker. Referring back to FIG.6, one will see that there is a typical prior art feedthrough capacitor140 shown adjacent the ferrule of the hermetic terminal 134. Asdescribed herein, prior art feedthrough capacitors 140, for primary EMIfiltering of AIMDs, have always been built from dielectricmaterials >1000 k.

FIG. 56 shows a circuit board 130 similar to the circuit board 130previously illustrated in FIG. 6. However, in this case, there is aquadpolar hermetic terminal 154 and there are four MLCC chip capacitors410 a through 410 d. These chip capacitors are low ESR chip capacitorsin accordance with FIGS. 51 and 52. In accordance with the presentinvention, these extremely low ESR chip capacitors will draw a greatdeal of RF energy from the implanted lead when it is in an MRIenvironment. It is important that this RF energy be efficientlydissipated to the AIMD housing 124 where it can be dissipated as RFenergy and heat. One will notice that there is a wide low inductanceground circuit trace 182. Therefore, between each one of the quadpolarleads 136, there is an MLCC chip capacitor connected to ground. Thisefficiently diverts RF energy from the lead conductors 136 to the groundcircuit trace 182. There is an RF grounding strap 264 shown which isrelatively wide compared to its width. This greatly reduces theinductance and makes it more efficient at high frequencies for divertingthe RF energy to the AIMD housing 124. In general, in anotherembodiment, the width of the strap will be >4 times its thickness.

FIG. 56A is a schematic diagram of the MLCC primary work horse filtersillustrated in FIG. 56. Shown are the work horse low ESR capacitors 410a through 410 d. In series with each one of these capacitors is shown aparasitic inductance Lp. This inductance results from the inductance ofthe leadwire from the length of the leadwire from the point of leadwireingress through the hermetic seal to the circuit board connection to thecapacitor 410. It also includes the inductance of the circuit trace 182and the ground strap 264. It is desirable to keep this parasiticinductance as low as possible. That is why the circuit traces 182 arerelatively wide and the ground strap 264 is relatively wide. It is alsoimportant to minimize the inductance of the lead between the hermeticseal and the active end (non-ground end) of the primary filtercapacitors 410. This could be done by making the wire shorter, larger indiameter or flat/rectangular or a combination of all of the above.

Referring once again to FIG. 56, one can see that the leadwires 136 aredirected to via holes in the multilayer of circuit board 130. There is ametallization about the via hole to which the capacitors 410 a through410 d are attached on the left side (or active electrode side). Theleads pass through the via in the circuit board 130 to another layerwhere they contact active circuit traces, which are shown as dashedlines (hidden lines). This completes the circuit from the leadwires 136underneath the primary filter capacitors 410 and underneath the groundcircuit trace 182 where they connect to other via holes (not shown).These not shown via holes would be connected to other AIMD electronics,such as an ASYC electronics chip or the like.

FIG. 57 is very similar to FIG. 56 except that a diode array 181 hasbeen added. It is very common in the input circuitry of AIMDs to providean overvoltage diode protection array mainly against the use ofautomatic external defibrillators (AEDs). AEDs can induce a very largehigh voltage pulse into implanted leads and this high voltage can beundesirably directed toward sensitive AIMD electronic circuits. Thediodes protection pack 181 provides high voltage over protection betweeneach one of the quad polar leads 136 and to ground or AIMD housing 124.

This is better understood by referring to the schematic diagram of FIG.58 which represents FIG. 57. One can see that there still is acontinuous electrical circuit connection 136 a to 136 d from the low ESRhigh RF-energy dissipating capacitors 410 a through 410 d of the presentinvention. Again, in each case the capacitor 410 remains the work horseas the primary diverter of high frequency MRI RF energy to the casehousing 124. Note that the circuit ground symbols in FIG. 57 are allconnected to the AIMD housing 124 through ground strap 264.

FIG. 59 is very similar to FIG. 58 except the high voltage protectiondiode array is shown on the other side of the low ESR capacitors 410 ofthe present invention. Since the high voltage protection capacitors arenot in series, but in fact in parallel, it would be well known to anyelectrical engineer that they could be placed anywhere along the lengthof the conductors 136 a-d. In either case, or in all of these cases, thelow ESR capacitor of the present invention 410 is always directlyconnected by way of continuous circuit pads through the hermeticconnector all the way through a lead conductor through to an electrodethat is contactable to biological cells.

Referring once again to FIG. 58, one will notice that the back-to-backdiodes 181 can clamp and shunt a positive or negative polarity pulse.For AEDs, it is common that there would be a biphasic pulse, meaningthat the pulse would switch polarity. Therefore, it is common practiceto orient the diodes back-to-back 181 so that may shunt energy with bothpositive and negative polarities. The diodes in the primary EMI low ESRprotection capacitor 410 also rule out the possibility of placing anyelectronic chips, active filters or other sensitive electronics in thisportion of the circuit path. These are the very components that would bedamaged by an over voltage or interfered with by EMI. So in general,these sensitive AIMD circuits are always downstream (to the right) ofFIG. 58.

FIG. 60 is taken from section 60-60 taken from FIG. 59. FIG. 60illustrates that instead of two discrete separate diodes 181 that arewired back-to-back, they also can be placed back-to-back in series asshown in FIG. 60. Sometimes these are called Transorbs©. In general,these diodes can be any type of transient voltage suppressor, varisters,avalanche diodes or Zener diodes.

FIG. 61 is very similar to FIG. 56 except that the RF grounding strap264 has been replaced with a simple leadwire connection 264′. This wouldwork okay at relatively low RF frequencies. For example, for a 1.5 Telsascanner, the RF-pulsed frequency is 64 MHz. Then as scanners haveevolved to higher and higher frequencies, the inductance of such a smallwire could become problematic. For example, there are many modernscanners in the market operating at 3 Tesla, which means that the RFfrequency is 128 MHz. The inductive reactance is equal to2×π×frequency×inductance. So if the inductance is small and thefrequency is large, one can get a great deal of inductive reactancewhich would make the diversion of high frequency energy through theprimary low ESR chip capacitors 410 less efficient. Another way ofsaying this is you really don't want anything in the ground path thatwould impede diverting the high frequency RF energy to the AIMD housing124. One way around this (not shown) would be to use multiple leadwires264′ thereby creating additional circuit paths to ground and therebyreducing the inductance.

FIG. 62 is very similar to FIG. 61 except in this case, the groundingleadwire 264″ could be routed directly off the ferrule 134 of thehermetic terminal subassembly.

FIG. 63 and FIG. 64 are very similar to prior art FIGS. 17 and 18 thatillustrated a flat-through type of feedthrough capacitor. These types ofcapacitors are unique in that the circuit current i₁ must pass throughits own electrode plates. The circuit diagram for such a capacitor 410is a feedthrough capacitor shown in FIG. 65. However, instead of havinga leadwire going through the center of the feedthrough capacitor, oneactually has the active electrode plates 144 of FIG. 18 going throughthe center of the feedthrough capacitor. In accordance with the presentinvention, the capacitor dielectric would have a k<1000, a very highelectrode plate count and an ESR generally <0.5Ω at the MRI RF-pulsedfrequency.

Referring once again to FIG. 63, one can see that this capacitor 410″would be installed on the circuit board of an AIMD a little differently.This would require a break in the circuit traces 178 a and 178 b, suchthat the circuit trace current allows current i₁ to pass all the waythrough it. It will be understood to those skilled in the art how tomake this simple modification to the circuit boards 130, as previouslyillustrated in FIGS. 56 and 57.

Referring once again to FIG. 63, one can see that there is a groundconnection 182 a and 182 b. This ground connection will be routed to theAIMD housing 124. As previously described, a preferable way to do thiswould be by means of a wide grounding strap which would minimizeinductance in the ground circuit trace. FIG. 66 is similar in outline tothe flat-through capacitor of FIG. 17.

However, FIG. 66 has a much different internal electrode arrangementthat is known in the industry as the X2Y attenuator. The X2Y attenuatorhas a line-to-line capacitor as illustrated in FIG. 70 as 410′ and 410″.For example, if lead conductors 136 a and 136 b were directed to theright ventricle (RV), they would be directed to a right ventricle distalelectrode and ring electrode. In a different implanted lead, directedinto the right atrium (RA), it could have another tip and ring electrodeconnected to conductors 136 c and 136 d. Capacitor 410′ and 410″ providea great deal of differential mode attenuation between the two lines. Forexample, if there was a large differential signal across the rightventricle lead, capacitor 410′ would divert that signal and prevent itfrom reaching AIMD electronics. Importantly, in accordance with thepresent invention, there are also capacitors to ground in the X2Yattenuator. These are labeled 410 a and 410 b. These (work horse)capacitors would be the one that primarily divert the high frequency RFenergy from the implanted lead conductors through the hermetic seal,through the diverter capacitors 410 a and 410 b, to the AIMD housing124. Therefore, in accordance with the present invention, the X2Yattenuator would be of a low k dielectric (no greater than 1000 k) andhave a very low ESR (<0.50) and have a relatively high number ofelectrode plates as this is particularly important for the divertercapacitors 410 a and 410 b.

Although several embodiments of the invention have been described indetail for purposes of illustration, various modifications of each maybe made without departing from the spirit and scope of the invention.Accordingly, the invention is not to be limited, except as by theappended claims.

What is claimed is:
 1. An active implantable medical device (AIMD),comprising: a) a conductive housing defining a body fluid side locatedoutside the conductive housing and defining a device side located insidethe conductive housing; b) an electrically conductive ferrulehermetically sealed to a housing opening in the conductive housing, theferrule having a ferrule opening passing through the ferrule between thebody fluid side and the device side; c) an insulator hermeticallysealing the ferrule opening; d) a conductive pathway hermetically sealedand disposed through the insulator between the body fluid side and thedevice side, the conductive pathway in non-conductive relation with theferrule, and the conductive pathway on the body fluid side connectableto an implantable lead conductor having at least one electrode; and e) afilter capacitor disposed within the conductive housing on the deviceside, the filter capacitor comprising: i) a capacitance of between 10and 20,000 picofarads; ii) a dielectric body supporting at least two ofactive electrode plates interleaved with at least two ground electrodeplates, wherein the at least two active electrode plates areelectrically connected to the conductive pathway on the device side, andthe at least two ground electrode plates are electrically coupled toeither the ferrule and/or the conductive housing; iii) wherein thedielectric body comprises a dielectric constant less than 1000; iv)wherein the filter capacitor is configured for EMI filtering of MRI highRF pulsed power by a low equivalent series resistance (ESR), wherein theESR is the sum of a dielectric loss plus an ohmic loss, wherein the ESRof the filter capacitor at an MRI RF pulsed frequency or range offrequencies is less than 2.0 ohms.
 2. The AIMD of claim 1, wherein thefilter capacitor is disposed adjacent to either the insulator and/or theferrule.
 3. The AIMD of claim 2, wherein the filter capacitor isselected from the group consisting of a monolithic ceramic capacitor, aflat-through capacitor, a chip capacitor and an X2Y attenuator.
 4. TheAIMD of claim 2, wherein the filter capacitor comprises a feedthroughfilter.
 5. The AIMD of claim 1, including a circuit board or substratelocated inside the conductive housing on the device side of the AIMD,wherein the filter capacitor is mounted to the circuit board orsubstrate.
 6. The AIMD of claim 5, wherein the filter capacitor isselected from the group consisting of a monolithic ceramic capacitor, aflat-through capacitor, a chip capacitor and an X2Y attenuator.
 7. TheAIMD of claim 6, wherein the circuit board or substrate is adjacent andattached to either the ferrule and/or the insulator.
 8. The AIMD ofclaim 6, wherein the circuit board or substrate is distant from andunattached to either the ferrule and/or the insulator.
 9. The AIMD ofclaim 6, wherein the filter capacitor is a first capacitor element of amultielement broadband lowpass filter having at least one inductor, themultielement broadband lowpass filter forming one of the groupconsisting of a reverse L filter, an LL, a Pi and an n-element lowpassfilter.
 10. The AIMD of claim 1, wherein there are no other filterscapacitors or electronic circuits containing filter capacitors in theconductive pathway between and to the insulator hermetically sealing theferrule opening and the filter capacitor.
 11. The AIMD of claim 10,including an inductor or inductance electrically in series with theconductive pathway, the inductor or inductance disposed before thefilter capacitor.
 12. The AIMD of claim 10, including a diode orback-to-back diodes connected at a first diode end to the conductivepathway and connected at a second diode end to either the ferrule and/orthe conductive housing.
 13. The AIMD of claim 10, including a diode orback-to-back diode connected in parallel with the filter capacitorbetween the conductive pathway and either the ferrule and/or theconductive housing.
 14. The AIMD of claim 1, wherein the at least twoactive electrode plates of the filter capacitor are electricallyconnected to the conductive pathway devoid of any intermediateelectronic circuits or filters disposed in series between the conductivepathway and the at least two active electrode plates.
 15. The AIMD ofclaim 1, wherein the filter capacitor comprises an element of amultielement broadband lowpass filter having at least one inductor, themultielement broadband lowpass filter forming one of the groupconsisting of an L filter, a reverse L filter, an LL, a reverse LL, a T,a Pi and an n-element lowpass filter.
 16. The AIMD of claim 1, wherein adielectric loss tangent measured in ohms at the MRI RF pulsed frequencyor range of frequencies that is less than five percent of the filtercapacitor's ESR.
 17. The AIMD of claim 1, wherein the filter capacitoris a passive component lowpass filter.
 18. The AIMD of claim 1, whereinthe MRI RF pulsed frequency or range of frequencies comprises 64 MHz or128 MHz.
 19. The AIMD of claim 1, wherein the capacitance is between 350and 10,000 picofarads.
 20. The AIMD of claim 1, wherein the filtercapacitor's ESR at the MRI RF pulsed frequency or range of frequenciesis less than 0.5 ohms.
 21. The AIMD of claim 1, wherein the filtercapacitor's ESR at the MRI RF pulsed frequency or range of frequenciesis less than 0.1 ohms.
 22. The AIMD of claim 1, wherein the dielectricconstant is less than
 200. 23. The AIMD of claim 1, wherein thedielectric constant is less than
 90. 24. The AIMD of claim 1, whereinthere are at least ten active electrode plates interleaved with at leastten ground electrode plates.
 25. The AIMD of claim 2, including aninsulative washer between the filter capacitor and the insulator and/orferrule.
 26. An active implantable medical device (AIMD), comprising: a)a conductive housing defining a body fluid side located outside theconductive housing and defining a device side located inside theconductive housing; b) an electrically conductive ferrule hermeticallysealed to a housing opening in the conductive housing, the ferrulehaving a ferrule opening passing through the ferrule between the bodyfluid side and the device side; c) an insulator hermetically sealing theferrule opening; d) a conductive pathway hermetically sealed anddisposed through the insulator between the body fluid side and thedevice side, the conductive pathway in non-conductive relation with theferrule, and the conductive pathway on the body fluid side connectableto an implantable lead conductor having at least one electrode; e) afilter capacitor disposed within the conductive housing on the deviceside, the filter capacitor comprising: i) a capacitance of between 10and 20,000 picofarads; ii) a dielectric body supporting at least twoactive electrode plates interleaved with at least two ground electrodeplates, wherein the at least two active electrode plates areelectrically connected to the conductive pathway, and the at least twoground electrode plates are electrically coupled to either the ferruleand/or the conductive housing; iii) wherein the dielectric bodycomprises a dielectric constant less than 1000; iv) wherein the filtercapacitor is configured for EMI filtering of MRI high RF pulsed power bya low equivalent series resistance (ESR), wherein the ESR is the sum ofa dielectric loss plus an ohmic loss, wherein the ESR of the filtercapacitor at an MRI RF pulsed frequency or range of frequencies is lessthan 0.1 ohms, wherein the MRI RF pulsed frequency or range offrequencies comprises 64 MHz or 128 MHz; v) wherein a dielectric losstangent measured in ohms at the MRI RF pulsed frequency or range offrequencies that is less than five percent of the filter capacitor'sESR; and vi) wherein the filter capacitor is selected from the groupconsisting of a monolithic ceramic capacitor, a flat-through capacitor,a chip capacitor and an X2Y attenuator; and f) a circuit board orsubstrate located inside the conductive housing on the device side ofthe AIMD, wherein the filter capacitor is mounted to the circuit boardor substrate, and wherein there are no other filter capacitors orelectronic circuits containing filter capacitors in the conductivepathway between and to the insulator hermetically sealing the ferruleopening and the filter capacitor mounted to the circuit board orsubstrate.
 27. The AIMD of claim 26, wherein the circuit board orsubstrate is adjacent and attached to either the ferrule and/or theinsulator.
 28. The AIMD of claim 26, wherein the circuit board orsubstrate is distant from and unattached to either the ferrule and/orthe insulator.
 29. The AIMD of claim 26, wherein the filter capacitorcomprises a first capacitor element of a multielement broadband lowpassfilter having at least one inductor, the multielement broadband lowpassfilter forming one of the group consisting of a reverse L filter, an LL,a Pi and an n-element lowpass filter.
 30. The AIMD of claim 26,including a diode or back-to-back diodes connected at a first diode endto the conductive pathway and connected at a second diode end to eitherthe ferrule and/or the conductive housing.
 31. The AIMD of claim 26,including a diode or back-to-back diodes connected in parallel with thefilter capacitor between the conductive pathway and either the ferruleand/or the conductive housing.
 32. The AIMD of claim 26, wherein the atleast two active electrode plates comprise at least ten active electrodeplates, and the at least two ground electrode plates comprise at leastten ground electrode plates.
 33. An active implantable medical device(AIMD) system, comprising: a) an implantable lead conductor having atleast one electrode configured to be connectable to biological cells ortissue; b) an active implantable medical device, comprising: i) aconductive housing defining a body fluid side located outside theconductive housing and defining a device side located inside theconductive housing; ii) an electrically conductive ferrule hermeticallysealed to a housing opening in the conductive housing, the ferrulehaving a ferrule opening passing through the ferrule between the bodyfluid side and the device side; iii) an insulator hermetically sealingthe ferrule opening; iv) a conductive pathway hermetically sealed anddisposed through the insulator between the body fluid side and thedevice side, the conductive pathway in non-conductive relation with theferrule, wherein a distal end of the conductive pathway is detachablyconnected to a proximal end of the implantable lead conductor; and v) afilter capacitor disposed within the conductive housing on the deviceside, the filter capacitor comprising: A) a capacitance of between 10and 20,000 picofarads; B) a dielectric body supporting at least twoactive electrode plates interleaved with at least two ground electrodeplates, wherein the at least two active electrode plates areelectrically connected to the conductive pathway on the device side, andthe at least two ground electrode plates are electrically coupled toeither the ferrule and/or the conductive housing; C) wherein thedielectric body comprises a dielectric constant less than 1000; and D)wherein the filter capacitor is configured for EMI filtering of MRI highRF pulsed power by a low equivalent series resistance (ESR), wherein theESR is the sum of a dielectric loss plus an ohmic loss, wherein the ESRof the filter capacitor at an MRI RF pulsed frequency or range offrequencies is less than 2.0 ohms.